I am too shy to ask this girl I like out or even go up and introduce myself or talk to her. What do I do?

I go to gymnastics, I am 15 and this girl looks about my age and she is stunningly beautiful. She is not in my group but we are near eachother sometimes. I go to gym once a week for two hours and have been going for two years and still I am too shy! I can't even go up and say hi or introduce myself or talk or anything! What should I do?