It is difficult for a boy cause he always has to take the first step and he can get a big NO for an answer. and if you like a girl a big NO is hard to take.
You can try first to be friendly. There are several ways.
1) ask her a question; ex.. Did you see the film on tV yesterday?
2) tell her something nice. ex .. I lke your hair today
3) ask her for something... ex. Can I use your pen?
After that, begin small things like sitting close to her, brushing against her hand and things like that.
Check on her reactions- if she doesn't like it ,stop immediately ,act cool,and just stay friendly.
If she seems to be ok, continue that way until you can ask her to go out. She might react positively one day and negatively the next - whatever you do, don't get aggressive, like some boys do.. you'll end up losing..in all cases, fake indifference.
Go slowly, you won't make a mistake .