Just say hey, how you doing? and then take it from there, don't try and plan the conversation in advance because it won't work.

just be like, 'hey, how are you?'

Don't always plan a conversation, it will sometimes backfire.

Start off saying hey, how are you, and whatever then move on to a topic you want to talk about.

Hi, I ate your cookie. :cool:

dont go all gangster with 'yo' or 'sup'
i never know how to reply to stuff like that :o

With yo and sup I usually say yo or hey back...then decide whether to start a conversation or not.

Say hey and ask her how she is. That means you're interested in her, not just looking for someone to take up time. But you're also not creepily invested in her. When she answers, don't just ignore her answer and start going on about yourself. I know plenty of people that do that, and it's a huge turnoff. Usually I'll just stop talking to them within a few minutes.

~Maggot

It all depends on how much you know this girl.. "Hey, what's up" is fine.

Unlike everyone else I think you should have an idea of something to say if nothing is happening. Have a back up thing to say that is interesting incase you both say hi and then fuck all happens.