"Wanna" is not actually an English word. It comes from the verbal contraction of "want to" and is a slang term when written like this. As people speak fast, "want to" can sound like "wanna" and has since been written as such in so-called 'text-speak'. (When text messages had to be short, this was a way of saving characters.)

wanna = want to used only in Usa englisch.