I fully agree with Peter McK and Robert.
Stay away from slang until you know proper English, or you are very comfortable with a particular expression. With slang, if you feel the need to ask when to use it, it simply means... don't use it!
Something like 'wanna' sounds natural only on the lips of a native speaker who uses it very naturally, and knows when to use it and when not to.
On the lips of a non-fluent speaker it usually makes the person sound like they have been learning English from a teenager or from a teenager show on TV. It just lowers the level of your English very significantly, and I strongly recommend to avoid it. It does not sound cool, it sounds low and cheap.
In writing it simply looks terrible no matter who is writing.