i mean what do I do and say to let her know how should I flirt with her.
Most Helpful Girl
It really depends on how well you know the girl.
If you have a class with her, try befriending her first.
Smile at her when you see her, or pass her in the hall.
If she seems to need help with class work, offer to help her.
If you don't know her and see her around, smile and wave to her.
If you're already acquaintances/friends then laugh at any jokes she might make, offer to do things for her, help her out, offer your number and text her just to ask her how she's feeling, etc.
There are a number of ways to convey interest, but the best way would to be upfront about it and just tell her, and see how she reacts.