From the Wikipedia entry:
Outside of Japan, manga usually refers to a Japanese comic book and mangaka refers to the author of the manga, who is usually Japanese. As of 2006, about 3000 professional mangaka were working in Japan.
Some artists may study for a few years at an art college, manga school, or take on an apprenticeship with another mangaka, before entering the world of manga as a professional artist. However, there are some that just start in manga, without being an assistant by applying to contests that various magazines run. For example, Naoko Takeuchi won such a contest sponsored by Kodansha, and Osamu Tezuka started out without being an assistant.
A mangaka will generally rise to prominence through recognition of their ability when they spark the interest of various institutions, individuals or a demographic of manga consumers. For example, there are various contests which prospective mangaka may enter, sponsored by some of the leading manga editors and publishers in the field. They are also recognized for the number of manga they run at one time.
... The -ka (家) suffix implies a degree of expertise and traditional authorship. For example, this term would not be applied to a writer creating a story which is then handed over to a manga artist for drawing.
I had thought of making mangaka a Cool Word of the Month, but there are too many "-ka"/"-jia" to make that practical. For example, a writer is called a zuojia (作家). jia is roughly equivalent to the suffix "-ist" (although it also corresponds to "-er"/-or, as the infinitive form may vary in Asian languages).