I would start with this issue here. It is like a brand new start for the hulk and has so far been a really great story. Someone did something with the hulk that no one had really done before and its going to leave the door open for something really big when its done. But you can start here and not worry about much.
This issue is actually the true start to planet hulk storyline going on now. Its not quite as interesting but it will put you where you need to be and make you understand why planet hulk is happening and why the hulk is going to do what he does when its over (I cant give away any details).