Well you could start by saying... "hello...you look really pretty" "I like your _____." Start a conversation. Something you both enjoy. And then somewhere in that conversation you could smoothly ask her out.
You could just be all straight forward and tell her you like her and you'de love it if she'd go out on a date with you. But don't pressure her... She then may feel on the spot and say no because she's shy.
You should start up a conversation with her! Ask her if you could walk her home or to her next class, then once you get to the destination ask her out on a DATE! To a dance or the movies. Confidence is key in this type of situation. Be confident and she will feel that and she will say yes! try being friendly at first. Don't just walk up and say 'hey you wanna be my girlfriend' flirt and let her know you like her first, then approach the dating topic :)
give her a compliment... then start a little convorsation. ask her if she would like to go out to eat, the mall, or maybe the movies. nothing fancy, us teens like low cost casual stuff. [take this from me, im a 13 year old girl] an goodluck bro!
Well start a convosation like hey, do u wanna do something some time. Or ask her to go to the park for a walk or take her out, make sure she knows your intrested in her and dont give up on hur (thats if u truly luv hur) then when ur starting to become good friends ask hur out or say well we have been friends for quite a while now, do u wanna take it a step furthur? If she says no, dont fret, stay her friend and then maybe try in a couple of years later.