Is there any type of schooling you have to take?
Not required, but there are "Manga Schools" in Japan (I saw them advertised sometimes in magazines). There is a difference between simply drawing pictures and drawing comics with stories to them. I’m sure there are also several techniques to learn. Many times, a Mangaka will make her debut with a magazine, and then proceed onto Manga-school. If you look at a Mangaka’s debut work and compare it with something she has written ten years later, the differences in art style and technique and formatting are dramatic. Those of you familiar with Watase Yuu’s work, check out this drawing she did for the cover of her very first manga. O_o
I know how to make my "debut" now, but then how do I keep it up?
This is a crucial time which depends almost solely on your popularity (with the editors/staff of the magazine you write for, and the readers.) Readers of the magazine write in to the staff and tell them what they thought of a certain Mangaka’s debut work. If the new Mangaka found favor with the editors and readers, she stays. If she doesn’t, she leaves.
I’m still not too clear on this part yet, but I believe once you make your "debut", you’re sort of signed on to do more episodes until you’re not popular anymore. A Mangaka might have spent 3 years thinking over and perfecting her debut work and it might have been spectacular… this is nice and all, but she will never be able to produce 30 pages per month if that’s the way she works. You’re basically put "on trial" during your first few Yomikiri (short stories).
When do I get my first serial?
Usually after you write about 6 Yomikiri, you are then assigned a Rensai (serial comic). [Though some Mangaka end up writing mostly Yomikiri, especially if you write for a branch magazine like "Shocomi-Cheese", or "LaLa"…] Your first serial is usually not very long (around 2-4 episodes). So if you have a story you really want to write, it’s a gamble. You can either use it as your first serial and have it much shorter than you wanted, or you can save it for later (if there is a "later", that is… THAT’S the risk… ^_-). Usually, when you are assigned a serial, the title page(s) of your first episode are in color.
When do I get my first tankoubon published?
Usually a Tankoubon of a beginning Mangaka (volume of manga about 180 pages long) contains her first serial, her debut work, and another 1 or 2 Yomikiri (depending on how long her serial and yomikiri are). Your first tankoubon is another crucial part of your career. The decision lies in the hands of potential manga-buyers (yes, even us overseas). If your first tankoubon does not sell well, and if you stop receiving favorable comments from the readers of the magazine and the editors, you pretty much fizzle out there. ^_^;;
What if I make it safely past my first tankoubon?
Then you continue to write more yomikiri and short serials, which are then published into more tankoubon.
When do I get to write my first long serial? (At least 3 tankoubon long)
This really depends on how popular you are and how well you sell. But it’s very hard to plan ahead for these things. The magazine reserves the right to say "the next episode will be your last" or "I want you to extend this story another 2 episodes". Most Mangaka really have no control over how long their stuff is. If some manga seem choppy or drawn-out too much, that is the fault of the staff on the magazine (and the fault of the readers for liking/disliking the story too much ^_-). I remember Mizuto Aqua mentioning that her Go! Virginal Hanayuukishould have been only 6 episodes long (thus constituting one tankoubon), but after she finished episode 5, it was extended 2 episodes ^^; I think she handled the sudden change very well, but it must have been mind-wracking to have to change her whole plan on such short notice.
Once you have been writing manga for a few years and have some loyal followers, you have a little more freedom in what you write and how long/short it is. It is then that most Manga-ka will bring out the story they’ve been wanting to write for years.