You know what they say: Knowing what to say is easy. Knowing what to say next is the hard part.
Since you aren't having any problem opening, you could expand on your actual opener a bit more. If your opener was "Hi" then find something else to talk about really quick.
If you are confident and have your body language down pat, then just tell her you noticed her/thought she was attractive and you just wanted to know if there's more to her than a pretty face.
You don't have to stick around too long, so use a false (or actual) time constraint (FTC), get her number and follow up with her later on.
5 minutes is all it takes.
Also you could use the "Goodbye" opener. Which is basically just combining your FTC and your opener into one. Just go over and tell her you were about to leave (which you should afterward because saying this then sticking around isn't going to make sense to her) but that you noticed her and just wanted to say hi and introduce yourself (I've used this before and just said "So next time we see each other we won't be strangers
. You don't have to go for the # close then, because if you are on a college campus or see her at the same place usually then you will probably see her again. The art of conversation requires you to be quick on your feet, original, genuine, and show a bit of finesse'.
It's something to keep working on, so try for a 1 minute conversation, then 2, then 3... you'll be captivating your sets in no time.