Make Eye Contact
When you spot someone you'd like to get to know better, one of the best ways to show your interest is with prolonged eye contact. This is more than a quick glance and looking away. When she looks your way, make sure she notices you looking at her. Hold her gaze for several seconds and then look away. If you keep staring or gaping, she might feel uncomfortable. Glance her way every now and then and if she's looking at you every time you look up, she might be interested in a conversation.
Smile and Laugh
If you're having fun with someone, the easiest way to show it is to smile and laugh. Don't create a fake laugh; people can usually tell when someone isn't sincere. Just maintain an easy smile to show you enjoy her company. If she says something you find amusing, laugh in your normal, natural way.
Listen More than You Talk
You want to learn more about the girl you like, so listen to her instead of dominating the conversation. It's flattering to have a person's undivided attention, so by listening to what she likes, doesn't like and what type of person she is, you show her that you're interested in her.
A sincere compliment is appreciated by just about anyone. There's no need to lay it on thick and gush. Find something you like about her appearance and compliment her on it in a nice, non-leering fashion. However, you probably shouldn't tell her she has a "nice chest" or "great butt."
Subtle compliments include how good her hair looks or smells, how pretty her face is or how great she looks in her outfit.
Get Close But Not Too Close
Leaning toward a girl to show interest is a part of flirting, but don't touch her if she doesn't touch you first. Some people don't want to be touched until they feel more comfortable with you, so unless she makes the first move, keep your hands to yourself.
You can still get close without making her nervous. Speak in a low tone so that she has to lean toward you to hear you better. You can brush her knee with yours if you're sitting close.