HELP! I can't talk to girls!?

Ugh! Every time I see a pretty girl and have the chance to talk with one I either (a) freeze and never approach her or (b) just play it too casual like I'm not interested ... BUT I AM! Or talk about something uninteresting and bland...making the conversation short. WHATS WRONG WITH ME! I can't pick up girls! I am my own co*k block!