Mangaka
Some artists may study at an art college, manga school, or take on an apprenticeship with another mangaka.  They then enter the world of manga as a professional artist. However, there are some artists that just start in manga.  One way to do this is by applying to contests that magazines run. For example, Naoko Takeuchi, author of Sailor Moon, won such a contest sponsored by Kodansha.  Osamu Tezuka, creator of Astro Boy, began his career without being an assistant.