People in the US write "wanna" in phone texts, IMs, and other informal communication. It is also used in written dialog, if this is how the character speaks. It is not acceptable in formal spoken or written English. Other similar slang words: gonna = going to / will dunno = don't know Sample text exchange: Person A: "You gonna be at the party?" Person B: "I dunno. Got homework."

it seems like wanna can't be used in written English ,just for short in oral English

Wanna means want to "As Nasty As They Wanna Be"