I continually think about an amazing piece of advice I once read- "If you talk to the girl, the worst case senario is that you walk away empty handed, no phone number, no date. If you don't go up to the girl, you're walking straight into that worst case senario without even doing anything!"
Whenever I see a girl I want to approach, if I ever feel anxious, I repeat these words in my head.
If you see a girl smiling at you, or better yet staring at you, it should be a no-brainer. Chances are she already kind of likes you, so it's unlikely you'll say anything to change her mind. So just go up to her, and make conversation. Observe the situation you're in, and play off it. Just have fun, that's the key, don't think of it as a stressful situation!