First of all, the major rule of thumb is is that you can’t be intimidated, you can’t be afraid, if you’re afraid you’ll probably say something idiot and look stupid. We all my look stupid or act stupid the first time we meet someone, just be casual and yourself, the worst thing you can do to yourself is be something you are not, that just ruins everything. Don’t act like a creep either, don’t come up behind her and like start getting into her discussions with friends or whatever.
It’s hard to give advice because you gave us so little information; how old are you, in high school, college, what? Do you see her around town, or only at school/work or what?
details, we need some more details to work with!