[Nyororin]

I say that instead of trying to learn informal Japanese and risking screwing up - just use what you have already learned and then take hints from your friends. It will be safe to talk to them as they are talking to you, so listen and learn. This will give you a much more natural sound in the end.

No one will think badly of you for sounding too formal, and it`s likely that they`ll laugh and tell you if it sounds weird. Better this than try to learn slang or the like from a book and end up sounding extremely odd.