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 Manga corresponds to the medium of art the artist uses: comics, or Japanese comics, depending on how the term is used inside or outside of Japan.
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. The Japanese term for such a writer of comics is gensaku-sha.
They also do basic editing and story suggestions of the manga itself. The influence of the editor can range manga to manga and project to project as both the mangaka and the company allows. Generally they are considered the boss of the mangaka and oversee the majority of the production to make sure it runs smoothly.
Most often, assistants are responsible for the backgrounds and screentones in manga, while the mangaka him/herself draws and inks the main characters. While they are often employed to help with art, assistants almost never help the mangaka with the plot of their manga, beyond being a "sounding board" for ideas. Most full-fledged mangaka started out as assistants, such as Miwa Ueda to Naoko Takeuchi, Leiji Matsumoto to Osamu Tezuka, Kaoru Shintani to Leiji Matsumoto and many others. However it is also possible for an assistant to spend their entire career as such without making the jump to being a mangaka in their own right.