What is the proper response to 'what's up?'?

Alright, I HATE it when people says what's up, or sup, to me. Sometimes I don't really know how to respond. Like if somebody just comes up to me and says what's up, am I supposed to say hi, or just go on and tell them what I'm up to? Or if I say hi to somebody, and they say what's up, sometimes they just keep walking so it's not like they really want an answer. I feel stupid when somebody says what's up and I'll say oh nothing, what about you? and then they don't answer so I'm not sure if they meant it as a hi, or if they really want to know. Ugh. I'm sorry if this sounds really stupid, but it bothers me! Thanks :-]