What should I do if I fall for someone after one night stand?

Let them know, at least a little. Phone, don't email or text, to say how much you enjoyed the date and getting to know them a little. You'd like to get to know them better, if they're willing. Would they like to join you (preferably choose a group setting) and your friends who are going to (the beach, a gig, the mall or whatever).

Best to choose this group get together as it's not too intimate, which might seem too heavy after a first date. Or if you use the same journey to college or work, seek to spend a little neutral time that way. After this, try a second date.