Hi CoolMyth,
Few things to take note, location does matter. Best is that you
have to catch her without her friends around. Sometimes, you may
not know if her friends have some other intentions (for guys) and
it may backfire on you.
You just need to walk right up to her and say, "Hi, I feel you've
got a great smile and you seem like an interesting person. My
name's ****** and I would like to have your number to know you
better."
Sounds simple right? The trick is to make it sound as if you
couldn't care less if she rejected you or not. Whether you are of
average height or average looks is insignificant. If you can be
confident when you talk, you have already won half the
battle.
If she gives you some funny reason like she's attached, she don't
give her number to strangers, gently bow out of the situation and
don't pester. It shows that she isn't interested in you so just
move on. Good luck! 
SMokE
P.S. Oh, if she asks for your number instead of giving you hers,
prepare for nights of misery, guessing when she's going to call. If
she refuses to give you her number but asks for your number
instead, ask her for her e-mail add or MSN. At least you'd be in
control of the situation. 
