To be honest, I've always been wondering why someone wants his dog to roll over... Anyway to get back on topic, imo the person above me called Dr!ver has some good tips. I also have a dog but I've never trained him, I like him the way he is.
I once trained my dog to tip peoples cups over on command (don't ask why), only took 15 minutes and i didn't use food, but its probably better to use food, you will have to push on their back half for sit and move them back to the same place for stay but its not that hard, just be persistent.
Lol I've never been able to train a dog to good..I can get him to come when I call him and thats about it lol..O and to attack