Just don't put so much pressure on yourself. If you go up to her thinking, i'm going to ask this girl out, if she says know it'll be the worst thing in the world then you're bound to feel anxious.
Just treat it like any other social situation where you are just chatting to someone. If it helps you to relax beforehand, take a few deep breaths, and think about something else to distract you from what you are doing.
At the end of the day, what's the worst that could happen? She thinks you're a freak and kicks you in the nuts, it'll only hurt a little bit.