I saw a cute girl today and she smiled and me what should I have done

Talked to her, maybe.
Or just stalk her on facebook...

How old are you? At 12, you smile back and go on with life.

At 18, you walk over and talk to her, get her contact info

Smiled back and introduced yourself. How old are you both?

Just smile back... out of politeness,if you see her again and she smiles maybe go over and say hello...

She- smile
You- smile back, How do you do/ Hello!. I like your dress, that's looks good on you.
She- Thank you!
You- By the way my name is (not Ni4 your real name);). Yada yada yada! :)