I am having trouble finding movie files that are compatable with my ipod touch?

I know the easiest is to purchase from Itunes...however I have some on my computer that I have purchased in other formats..Which file do you use, or what program can you use to convert? I am actually getting frustrated to the point of taking the ipod back...This was one of the features I was really looking forward to using? I have tried wav, mov, mp4, and mpeg files HELP!!!