Most Helpful Girl
One way to get over fears is by facing them. Acknowledge the fear, accept it, but then take a deep breath and work through it. You'll realize once you do it several times that there's nothing much to be afraid of, but that first time is the hardest. You just have to force yourself to do it.
Work up to it if it's a really debilitating fear, but make that effort to overcome it. It doesn't happen magically. I've found that just telling yourself 'screw it' and doing it without giving yourself time to think about it or build up fear works well too. Good luck.
Most Helpful Guy
The more you do it, the less fearful you will be.
What's worse, having a girl who isn't interested in you turn you down (so why should you care anyway...), or being a coward who never gets what he wants in life because he's afraid to take action? Which should you actually be afraid of?
You need to get angry with yourself. If I don't do something because I'm nervous, I become furious with myself and force myself to do it. You need to learn to HATE being cowardly. You need to learn to hate your weaknesses and then do something about it. Don't get depressed and mopey, fix the weakness.
Start small. Get used to just making eye contact with girls. When you are comfortable with that, then add in shooting them a smile. Then saying hello. Then striking up a conversation. Then asking them for dates. Then asking them to sleep with you. Work your way up.