What does it mean when you start being nervous when you see a girl you like?

It means that you've started to become self conscious.

You're most likely worrying about what she might be thinking about you, or perhaps you're worried about how you might be coming across. Very common worries for both teenaged boys and girls.

It's a totally natural thing, but try not to let it completely control you like it does with some teenaged guys. Being totally happy with yourself is what's important, having other people be happy with you is a lot less important and more often that not- not important at all.