Aw. I am almost positive you are doing just fine. So long as you don't drool, or cackle maniacally, or, I don't know, moo, you're doing just fine. Obviously, you're approachable. There are alot of women out there who are human icebergs. They act like they grew up on the Ice Planet of Hoth. I bet you are a very warm, genuine women with a pretty smile.
Otherwise, the dudes wouldn't be coming up to you. College guys are clueless gorillas who drink beer and play with action figures. And most are just as shy as you. Remember that next time. It probably was a small act of courage to lumber up to you and compliment your necklace or just say "hi." Return the favor by being a little bit brave, and compliment him on something.
Flirting is like ping-pong. Just keep the ball bouncing back and forth. Compliment him, talk about the weather, ask him about his favorite television show. Talk about a crazy thing you saw online, crack a joke, or just ask him questions about him. It's really easy. Pretend he's like one of your female friends, up until the part where, maybe later, he leans in for a sloppy kiss that you don't even see and the next thing you know: his lips are wrapped around your nose.