The answer is very simple - never use "wanna," unless you want to sound like a teenager trying to "get down," with young people. In this context "get down with," means to deliberately act in a way that is intended to help you get on with somebody. "Wanna," is *not* English - It is text speak used by people that are too lazy to type "want to," on their mobile phones. If you are in any kind of Whatsapp English group then I suspect that you may have seen "wanna," in messages within this sort of group?

You just need to recognise "wanna" as a slangy form of English. There is a place for it, but if you want to speak English as well as you possibly can, then use "want to". Also, keep in mind that native English speakers who use "wanna" are fully aware that the correct form is "want to". Don't ever write "wanna" if you want people to take you seriously.

"wanna" is used very informally in text speech, however, it's important to bear in mind that in standard conversation, many English speakers pronounce "want to" as "wanna", not just young people.