Just call. I was like you back in high school but slightly better. You must call! Even if you don't succeed this time, you will learn from this experience and grow as a man. The next time you like another girl you will have the courage to do it because you've had previous experience. It was extremely difficult for me to call a girl I liked the first time, but after that, it was still difficult, but I had the courage to do it.
Ok, this may sound stupid, but try it, it might help, a friend of mine did this coz he was also a wimp. He writes down what he was going to say before he calls, hahaha! He still wound up sounding like an idiot when he was on the phone.
Good luck!