Never, ever, ever write "wanna" anytime, anywhere. Period. (Or "full-stop," according to your personal preferences

)
It looks illiterate when used by a native speaker and absurd when used by a non-native speaker.
Having said that, the above postings are correct that it is a contraction of want + to. You will often see it in the lyrics of pop songs (as when the Beatles famously described wanting to hold your hand). This is a special, permissible usage to which pretty much nobody objects. You will also occasionally see it in internet-based chat rooms or in text messages (SMS). Beyond these special instances, this form is not considered Standard English and should be avoided like the plague--by non-native speakers and native speakers alike.
Fragen?
Don