You like this girl and you want to get to know her. How do you get her phone number without making her think you want her to be your new girlfriend?

[edit] Steps

Talk to her in person. If you talk to her, you will increase the chance of getting her number much faster. Make sure that if you do touch her, you make it a very friendly, non sensual type touch. Refrain from touching her hand or face.
Get to know her. Ask her what her favorite band/musician is, her favorite food, her favorite shop, etc. If you get to know her, she will also get to know you as, hopefully, a nice and pleasant person.
Enlist the aid of her friends. Ask them what does she like and dislikes. Getting to know her friends, especially if they like you, can really help you out here. Be sure you let them know that your only interest in any of them is platonic and that you aren't trolling for dates, or trying to get over on them in any way.
Ask for the number. Talk to this girl and chat on a simple, yet nice topic about: movies, bands, summer vacation, school, etc. Then tell her you would love to chat and hang out from time to time, and then ask for her cell/home phone number. Say that you think this is the beginning of a wonderful frienship, because you see her as a wonderful friend. You should have no trouble getting that number.

[edit] Tips

Look neat and nice. Smell good and look good. This will make you look 'approachable' to the girl and will allow the girl to talk to you.
When she talks, listen. Don't just look at her and nod, waiting for your chance to leap in and tell a story about yourself. REALLY listen. This way, she'll think you are a good listener.
When you talk, talk about simple, ordinary stuff.