Well, let's see. It depends on the girl you're planning on asking out. If you want to ask out a slut to, ahem, "relieve some stress" you may be under, then all you really need to have is looks. They'll go out with just about any guy. Thus why they're called sluts.
But, if you're actually looking to get a decently respectable girl (or higher) you need to be yourself, considering that yourself is a tolerable person. If you're a little annoying pest that finds it funny to give wedgies or fart in people's faces, that'll get you the wrong attention. Avoid doing stuff that she doesn't like to get her attention, cause if you do, that's stepping all the way back to freaking preschool. Basically what you need is three things: humor, the ability to hold up a conversation and confidence. The third one is probably what you lack, seeing as you're asking for help on a forum. No offense. If you're naturally a funny guy, you're basically set. If you're naturally funny, and relaxed around girls, you're definitely set. Add that up with the ability to hold a conversation, and keep talking to this girl. You cannot expect to have a good conversation with her for five minutes and think she'll say yes if you ask her out. Just cause you like her, doesn't mean she likes you back. Yet. So, if you just keep talking to her, and being really friendly and nice (girls hate jerks), and ask her out after a while, you'll be fine. All you need to do is forget about the "what if"s, y'know? Thinking to yourself "what if she says no and calls me a creep?!" that'll just bring your confidence way dowwwwn. So don't.
Also, seriously don't ask her out out of the blue. A conversation that goes a little something like:
Her- "Hey!"
You- "heyy, what's up?
"Oh nothing much. I have so much homework I need to do, ughhh."
"That sucks, wanna go out with me sometime?"
"...huh?"
I suggest just being like "Listen, I have to ask you something." and then asking her out. It'll bring down the shock level by quite a lot.