Best way to learn any language is to immerse yourself in it completely.

But since most of us don't live in Japan or in Japanese communities, the best bet is to get as much material as possible that allows you to hear native speakers and just listen. Learning vocabulary out of a book is useful, too, but it's pretty weak unless supplemented.

Rosetta Stone is supposed to work great, too! Go to their website and try their free demonstration--I believe it's done in Flash, so you'll need that. But it basically takes you through a full lesson in Turkish. And it worked. I now know how to say "girl," "boy," and "man" in Turkish. This was from a five minute demonstration O_O .

Also... learning at high school/university etc. is probably better too. That way you are in a class with other people learning the language and you are being guided by the teacher who you can also ask for help from.

Amnell
 

A person should know turkish first you mean?
@Amnell