How Are You?

I dont know why foreigner always like saying "HOW ARE YOU?" when ppl meet each other. coz in china just say that when u and ur friends have long time no see! so when ppl ask me "HOW ARE YOU", I really dont know how to say is better, so just thinking thinking and thinking again, then said "er...i am ok" or "er...i am good" or "er...not bad" something like that, er......囧.....er....again!