Well, it would help your case ALOT if you were 25 and a virgin. No, I'm not kidding. If you tell a girl you have never had a girlfriend but you HAVE had sex, then she will think you're a "player" and aren't serious. Why did you have sex if you weren't dating the girl? It's really none of MY business, and you don't have to answer that, but I want you to think about it yourself. Seriously. But don't lie about it. What's done is done and honesty is very important.
And don't worry about being shy. I am a woman who happens to think shy guys are very sexy. I know there are other girls out there like me. Find one.
Be very open, very honest, and most importantly, don't just talk about yourself. Ask her questions about HER. Act like SHE is the center of the world. Be a perfect gentleman, treat her like gold and LISTEN to her. Be a friend, but don't be TOO nice or she might walk all over you later. If you're nice enough, the truly nice girls will probably ask YOU out. They'll respond to you if they think you're really interested but too shy to do anything about it.
Good luck!