If you want to be a professional manga writer what would you do?

Some guy told me I should send a manga company a one-shot. If they consider it a great manga, you can make your own manga. That's how many manga writers like Masashi Kishimoto (creator of Naruto) started out. If your manga is way popular, the manga company's boss or publisher would discuss with you if you would want it turned into an anime, I think. So you should think about finding the adress or manga company building. Its cool you want to make your own manga, because I want to make my own too! Good luck, have fun!
Tokyopop, a major manga publisher in North America, holds a contest for people to submit their own manga - it's called The Rising Stars of Manga. Some successful manga have spawned from this! You also may want to purchase a manga editor for your computer, such as Manga Studio 3.0.