Girls, how should I approach her for the first time?

I am a sopre in high school and I really like this freshmen girl. I am a shy person when it comes to meeting new people. I have had girlfriends in the past and talk to a lot of girls. It is just something about her that amazes me. She is very pretty and I think a lot of girls are pretty, but there is just something about her. Right now I am at the point to where I can't stop thinking about her. I day dream about her in a lot of my cles and I have gotten in trouble a few times for not paying attention. I see maybe 3 or 4 times throughout the day at school. Everytime I see her I can't help bit smile and it's really obvious because my friends tease me about it all of the time. I see her in the hallway, at lunch, and sometimes at her locker after school. I usually try to approach her while she is in line at lunch buying her food, but I always talk myself out of it. This is the only time I see her alone. I know where she sits at lunch but the problem is I don't know anyone she sits with. I would approach her at her lunch table but I don't know if her friends would accept me or tease me about it and embarr me in front of her. I know it should only matter what she thinks about me, but like I said I am shy and don't want to be teased and disliked by her friends. After school when I see her at her locker she usually has a friend with her and I do too. I've thought about asking my friend to be a "wingman" for me, but he isn't that type of person. It's driving me crazy that I don't know how to approach her. Please help. Thanks!