Start by drawing and writing manga. You can study at an art college or manga school. Then you can develop your artistic abilities and technique. You can see the difference training makes in the development of many Manga-kas' work.
Apprentice with another manga-ka. An apprentice assists with the drawings, maybe shading or erasing. Send a letter with samples of your work to a manga-ka whose style you admire.
Develop your area of expertise. Manga markets to specific demographics such as gender, age and fictional genre. For example, shoujo manga targets girls, shounen manga targets boys and josei manga targets young women.
Enter a competition for publication and recognition. If you are selected for "Rising Stars of Manga," your short story will be published in a manga volume. US manga magazines such as "Shounen Jump" or "Shojo Beat" offer competitions where you receive an award. Post your manga on a website like Tokyopop.com and develop a fan base.
Send your manga story to the editing department of various manga magazines. It may take several tries to be accepted. The writers, staff and readers of the magazine all make editorial comments. If you are popular, you then sign on to do more episodes on a trial basis.
Continue writing short stories, or yomikiri, for your magazine. If you are successful at that, then you will write a serial comic known as a rensai.
Write a graphic novel, or tankoubon. This is a separate volume of around 200 pages. This is an important part of your career. If your tankoubon sells well, your manga-ka career is on its way.