Just ask her out. the worse thing she could say is no right? I know not everyone is as outgoing as I am and i still get nervous when I ask guys out. But just hint at the fact that you like her if you're too nervous. If she picks up on any of them and seems interested, then that should make it easier.
To work up the confidence to ask a girl out, you need to overcome your anxiety. Go to social places where you will be exposed to them and know how to strike a conversation. Be prepared for any of her response and strive to maintain friendship. Check out more tips from http://www.confidencedomination.com/how-to-ask-a-girl-out/.
If you like a girl and you guys are friends or share common interests then you know that you would get along. Just talk to her and you will know when the right time to ask her out is. Once you get that feeling just go for it, 90% of the time she will say yes. GOOD LUCK :)