ok so Aang said that Avatar Roku was supposed to teach him to be the Avatar, well why doesnt Roku just teach him now? In order for Aang to be here, Roku would have had to die, hence the term "past life", so if Roku was somehow supposed to come back from the dead to teach Aang to be the Avatar, can't he just do it now?

I don't think you can actually teach the Avatar to be the avatar. I think he's suppose to be a guide, and tell him stuff (a la Avatar State)