Only when you are alone in your room with your head under the pillow!

You can pronounce "want to" as "wanna", but never write "wanna". Also, sometimes with non-native English speakers, when they try to say "wanna" or things like that it sounds forced and unnatural. You should probably just use want to for now.

Never. Do not use it. It is below slang and never written.

It is slang for 'want to.' It is very informal. So you could use it in the following examples: I wanna go to the shops Do you wanna go to the park?

You can use it when singing songs and you want to sound cool e.g. "I wanna hold your hand ...." by the Beatles!