Self confidence really can only be gained through experience - if you're not confident talking to girls, just keep trying until you're well at ease with them, don't worry about the consequences, don't get ahead of yourself, just think about what you're doing as you're doing it. It's a lot easier to talk to a girl when you're only thinking "hey, talking to cute girls is fun!" instead of "Oh gods oh gods what if she doesn't like me and what if she does and I bet she's look really great with no clothes but oh crap I just made a fool of myself and I've been staring at her for wayyy too long dang this is embarassing." *approximation of my train of thought talking to girls I liked when I was in junior high*
Once you've got all the clutter out of your mind, you can converse pretty normally and just speak your mind - girls usually don't get offended when you tell them you think they're pretty; just use common sense when you choose your wording! "That shirt looks really good on you" goes over much better than "zomg I can see ur boobz!"
After that, you're pretty much in - talk about common interests (assuming you have some,) talk to her about HER interests even if you don't share them, if the opportunity arises, tell her (briefly!) how great you are, and then if things seem to be going well, get her number and/or set up a date!
Still though, the main thing is to just do it. Yeah, you'll get shot down sometimes, but a wise man won't focus on the five that say no, he'll focus on the one that says yes!
