You can stop being nervous around the girl you like by taking a deep breath before talking to her. Then, try to think of her as a friend instead of what you feel for her which help you relax a bit. You should also look at her face when talking so that you don't get distracted. See http://www.wikihow.com/Be-Comfortable-Around-That-Special-Someone for more tips.
like clemence says you have to take a deep breath, concentrated and what I would do is to ask her something let her answer and then go from there that way you will b more relax and calm and think of her as somebody that you been knowing for a long time that way you will feel more confident and feel less nervous