Find a way to talk to a lot of girls. You need to get to the point where you don't care what they think about you, and you're just talking about something that doesn't seem threatening to you. Maybe get a job someplace where you sell stuff, and a lot of your customers will be girls. Perhaps you could talk to girls at church or in a club. Anything that lets you talk to them about stuff that doesn't force you to think up something to say. Once you can get past that, the rest will be a lot easier.
Maybe you should just act like your friends around the girls and just be your self. Close your eyes (don't actually close your eyes when you're talking) and pretend you're talking to your best friend. Unlike the answers above this one, this one is more of a... safe way. Just become friends with the girl, don't try to push becoming boyfriend/girlfriend. Girls are people, just like you and me, so it really helps to try to talk to them and let them know that your a nice person. The biggest mistake you can make is by asking her out right away. Trust me on that one. You need to become friends. I can't carry out a conversation well at all, but now I can actually talk to girls like their a regular person instead of someone that is unapproachable. Just be their friend, and eventually you'll stop being nervous. First, you have to get to know girls. Not in a boyfriend-girlfriend way, but as friends. Are you uncomfortable or awkward with your close friends who are boys? Probably not. It's the same with girls. Just get to know them, and be friends. Then eventually, you won't be nervous around them anymore. Pursuing a boyfriend-girlfriend relationship from the start is a big mistake because, to feel comfortable around a person, you need to get to know the person.