Depends. Have you actually tried to get a girlfriend? I didn't really date in high school because I didn't see the point. At that point, I was having a hard enough time figuring out what my place was in the world to deal with somebody else, too. A big problem people have is that they become addicted to relationships during high school. At that crucial stage of emotional development, they never really develop on their own, they always have somebody else there so they don't become self-reliant. I have a few friends who are like that now and they both had very serious relationships all through high school.
Once I got to college though, I went out there and found some girls I thought were cool and dated for a while. If that's what you're talking about, then, seriously, just be confident. Walk up to a girl you like, strike up a conversation like it's no big deal, since it really isn't, and then ask her to hang out sometime. It's MUCH easier than you think. The problem is that, with more introspective people, they tend to overthink the whole thing. They always wonder what'll happen if she says something or how to say it just right. So then, even when they do ask, they're second guessing themselves the whole time even if they don't know it. Just be cool and collected about it, she might say no, but hell, no big deal. Most girls don't have any opposition to hanging out and even a "no" might actually just be because it's bad timing. Be cool about it and shrug it off. No problems.