Manga
With the proliferation of manga all over the world, I have been receiving a lot of requests for information about working in the manga industry. I am no expert on the manga industry by any means so it would be best to check other sources.  I gave up on trying to work as a manga artist because it seemed like a really pathetic existence where you had to sell everything you had just to see it get made.
For anyone interested in publishing manga in Japan, my suggestion would be to publish where you are first.  
The Reasons Why
If you cannot finish your idea the way now then you will not finish it were you to move somewhere else.
  Moving to Japan will not unleash your creativity or spirit any more than moving to another city anywhere else in the world would. In fact, the confusion of moving will probably stall your creativity out for at least a few months. Once you get to Japan you will have to find a job, much as I mentioned in the section about animators, while you go to a school or try to find a job. Since a manga artist is 100% freelance (not a corporate employee) you will not have a sponsor for a visa so the only way you could get a visa to stay and work in Japan is by marrying a Japanese national. The laws are structured specifically to keep people who want to try and live freelance off the Japanese economy out of the country. Your drawing manga does not help the country in any way and it's something that anybody can do so they are not even going to consider granting you a visa for it.
 The vast majority of manga "studios" in Japan consist of one lead person and a couple assistants in an apartment somewhere. There are no big manga companies who hire artists and keep them in-house. It's not like the US comics companies at all. Everybody is freelance.
The small companies usually have a few beds, a lot of reference material, various art supplies and maybe a computer or two. Nothing special. You could do the same kind of work in Topeka or Buenos Aries or Luxembourg or Vladivostok.. Nothing is holding you back from doing it right now!!! 
Manga schools are a joke.
  They're more for kids who come out of high school and can't get into colleges/universities to pass time in until they can get a job doing something else. No famous manga artist is a graduate of a manga school. Art schools, yes but manga specialty schools, no. The things you learn in manga school are pretty much the same things you'd learn as an assistant. Good artists read everything they can and study manga, illustrations, photography books -- anything art related. (Studying manga alone will only weaken you.) The only way you can learn to draw is through lots and lots and lots of practice. The sooner you start that practice - like right now - the sooner you will get to the point where you can do what you want. You have at least 10,000 bad drawings built into your hands so work at getting rid of them.
The Plus Side
The biggest advantage you could get from working at a manga studio in Japan would be learning from the chief pro. If you get in at a good studio and you can stick with it you can learn a lot. In a few years you might be good enough to break out on your own.
A manga assistant mostly lays down tone, sometimes inks the panels, puts in motion lines, backgrounds and sound effect text. The rates of pay vary widely but it is usually around 80,000 yen (or about US $800) a month to start. Sometimes the first months are done on an intern sort of situation.
 If you come up with an interesting idea you submit it to the publisher. Although you can submit from anywhere you should be there for the review though as it is best to take that in person. You will have to complete quite a bit of work before they will trust you to do a series because if you can't handle it they are the ones who will be hurt most.
The Minus Side
If you cannot get published in your own country then it is extremely unlikely that you could in Japan. You may think you work in a manga art style or tell manga style stories but Japanese publishers will not. You will never be able to offer them manga style artwork superior to what they can get locally and you will have a severe disadvantage in the first place because of the communication problems. Working in an original non-"anime/manga" style would increase your chances of getting published by a huge factor.
 If your work is exceptional, which is what they are looking for, then you will be able to get published more widely, make more money and live a more comfortable life in the US or Europe. The best way of doing it would be to get published in the US and then see if you can get a Japanese publisher to pick your work up.
  If you make a manga that's basically a rehash of a bunch of elements from popular anime shows your story will fail in the Japanese marketplace. The take-off stories that do well do so because they have original twists and are also based on a great knowledge of the source material. Much of the work shown to me by Western artists who want to do manga is very derivative and this is usually because the artist is so excited about discovering manga that they don't see that they're "borrowing". (I was that way too.)  That's fine as a fan artist and maybe to publish small-press in the US but the Japanese publishers won't get past the first few pages before rejecting it.
 The publishers and readers will want to see something that they can't get from a native artist. This can be your main market leverage. Use it! Create something that only you can do. (And the Japanese readers are not interested in your version of Japan. More often than not they just dismiss you as being an ignorant outsider. Do your story about something else or publish your manga outside Japan.)
 Never forget that there are 200,000 wanna-be manga artists already drawing their own manga, many of whom are directly competing against you. This brings up a good point - if you come up with a good manga idea there is a good possibility that you can make more by selling it as a self-published book (doujinshi) at Comic Market than by having it picked up by some smaller publisher.
Pretty much everything I mentioned about the anime industry is similar here. 
 In order to be published in Japan your manga must be in perfect Japanese.
   The Japanese are extremely critical about the quality of written language and any mistakes or oddly written prose will get you labeled as an incompetent. If you cannot do it perfectly then you must hire someone who can. The publisher will not provide this service. You must also hand letter the sound effects, which means you must have a very good understanding of how the language is used. This sort of understanding can only come from life experience and no language school can help you with this sort of work. Maybe a minimum of 3 years living in Japan would be a good start.(See the notes in the animation section about this.)
  Manga artists don't make very much.
    Can't get a visa, can't get a loan, and landlords will be very wary about renting to you. You may have noticed that manga artists are often depicted as poor, starving nebbishes in anime (and manga) and Japanese magazines. That's because it's pretty true. The ugly truth is that most manga artists make about 10,000 yen (US $100) a page. The conventional manga wisdom is that you have to do at least 2 series unless you have a mega-hit. The first series pays the rent, utilities and staff and the second series pays the staff who make it and you.  
 In return for publishing your manga, the publishing company acquires all rights to it and most of the control over it.
    This is worth repeating yet again -  you do not control the story, the publisher does. They get 100% of the sales of the weeklies/monthlies and you get paid by the page no matter how many they sell. They control the content and the art. You write the story but they assign an editor to you to ensure the quality and acceptability of the product. If somebody picks it up for animation then it is the publisher who makes the deal because they own the property. You will not be involved in any way with the animation production. (The director can make something that is completely unlike your manga but has the same name. A manga artist I worked with had this happen. The story was the same but the characters were different somewhat and there were various things the writers put in that the manga artist didn't like. He also didn't like the character designs but had no say.)
 You make your money off royalties.
   The collected books (tankoubon) will be your salvation as you get 40% - 60% of the profit on them. If your manga is picked up for animation or a game you will get some off of that too. (You might get some back from merchandising as well. Depends on your deal.) These can be very lucrative, especially if you get a hit down the road and people go back and buy up everything in your library. If the show is a hit and they decide to animate other stories you've done that can make you a very good amount of money.
 You cannot reprint the work, you cannot sell original pages, you cannot even sell the books on a street corner by yourself. You cannot make the English, French, whatever language version and sell it because those rights also belong to the publisher unless they wish to relinquish them or you make some deal. 
What you usually retain is the original concept so if you want to go somewhere else and make a new series or produce new works based on that concept you can. This is a very important and very valuable right! Think of it as coming up with an idea for a new part for a car and having the car company pay you to produce the part. You own the patent and can go somewhere else and produce it but you can't own the parts already produced and you have no influence on how they're used in the car. That may sound pretty ugly when considering a manga but that's pretty much the way it is.
 The editor also keeps you on schedule. You need to produce however many pages a week, rain or shine, sickness or health, life or death.
　
Doujinshi
  Doujinshi are self-published manga, sold through venues like Comiket (Comic Market) and through mail-order and online. Retail bookstores almost never carry doujinshi.

  Doujinshi rarely generate enough income to survive off of unless the artist is well-respected or extremely popular. There's no possible way to get a visa if you're doing doujinshi. It would be like putting "gay porn author" on your application.
　
"How do I contact manga publishers in Japan?"
Their addresses are in the backs of the manga that they publish. You must contact them in Japanese (good Japanese) and they will usually not accept submissions directly. There are contests throughout the year where they accept submissions and it is best to check the weeklies/monthlies to find out when they are.
　
Summary
 These are the requirements for working as a manga artist in Japan:
  • Fluency in the Japanese language, both written and spoken (estimated 5 years of full-time study in Japan to attain this.)
  • Good writing skills - grammar, spelling, etc.
  • Living in Japan with own visa. See visa information.
  • Most publishers strongly prefer someone who has experience in the manga industry as an assistant. Recommended 2-4 years.
  • You must have an original style. If your work is "heavily influenced" by any popular manga or anime it will not be picked up because there are plenty of Japanese artists who already draw like that.
  • The knowledge that there are over 1,000,000 people directly competing with you to get their manga published by a major publisher.
  • Enough money to survive until you can make money from publishing or another job. (Note that if you have another job it will take most of your time and you won't have enough time to do your manga.) An estimate of costs follows:
    • Somewhere around $25,000 US. Based on 1 month's rent being $1000 (typical) x 12 + $4000 cost to get into an apartment + $500 living costs for one month (minimum) x 12 + about $3000 worth of materials. (This assumes you somehow learn the Japanese language in a year and somehow get a publishing contract in a year - something most Japanese people work for 5 - 10 years for.)
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Conclusion
My point here is not to scare you away from the manga industry but to get you started doing manga NOW rather than expecting that you will be able to do it once you get to Japan. Pitch your work to publishers in Japan and the US and elsewhere. But remember you must finish something, even if it is only a few installments. Nobody will pick up a concept on a promise.
 OK, enough! Quit surfing and get at it!