Mostly what Devie said. Also, I'm kinda like you. The shy guy that knows he's going to screw up before he even knows which girl he's going to ask out.
Anyway. The way it's mostly worked out for me is find a girl that seems as interested in you as you are in them. If all of the conversation ends up you asking about her, and it gets kinda awkward, that's when you know this isn't going to work out.
One time there was this girl that I used to hang out with. I noticed one day that she seemed rather detached from our conversations, and I was asking all the questions. So then I was like 'screw it.'
Also, try to make sure they're not already in a relationship. That happened to me one time, there was this girl. I asked her to the dance and she just said no without any details (of course she responded after the dance and she didn't even go lol :3).
Later, I found out she was with someone and kinda in my head I was like:
[spoiler]
[/spoiler]
But yeah, any of the successful relationships I've been in (I say successful, but eventually they ended one way or another) usually started that way.
[spoiler]Oh yeah, and band camp lots of girls have been giving me the eye. >.>
kinda creepy.[/spoiler]