I agree with Spiritus; asking for her number right off the bat will turn her off. She'll make any excuse not to give it to you: i.e. "oh, I don't talk on the phone", or "I have a boyfriend" (even when they don't). Take it from someone who used to do the cold approach a lot. It never worked and always made things awkward.
Much better is to approach with a compliment about some attribute they have like a tattoo, their glasses, rings, t-shirt design, etc ("Thats a pretty cool White Stripes shirt, have you seen them live?") Complimenting hair, nails and shoes is a bit iffy as she may think you're not straight and genuinely interested in it for yourself. By complimenting her, she'll be flattered and want to share some story about said item, which gets you going on a conversation, bonus points if you also have a background or interest in the item and a story you can share. The conversation will start flowing from there. Once you're ready to wrap up and get out of there, act like you're in hurry, "Hey I gotta go but I'd like to talk to you some more, text me your phone number okay?"
If you've reached this point and she's been talking with you without trying to blow you off (re: giving short answers to everything you ask: "YOU: "where'd you get those shoes?" HER: "Amazon." *silence*...) then you'll likely get it. It's all touch and go, but you got to get out there to get experience. Feel it out as you go, it won't ever be perfect, but if you do it enough you'll find success. Good luck, God bless.
P.S. -- A good thing to keep in mind: for many reasons, a lot of chicks get nervous talking to guys too: It could be that they're inexperienced themselves, or they're not used to being hit on, they don't like being hit on, or they're just shy. Don't take it personal, just try your best.
