The best advice I can say is be yourself. Men get stupid and I mean REALLY stupid when they like a girl.
Take me for example, the first girl that I really really liked (which sadly it didn't work out) I liked her for seven YEARS, and couldn't muster up the courage to tell her until sometime in the summer of 2010. Turns out she liked me too but didn't have the courage to confess her feelings either, so what did I do? After learning that I started talking to her... A LOT and finally decided to fly out to see her in California. Had a great time, but here's the part were the whole "Men get REALLY stupid when they like a girl"
She's the severely shy type, in fact she was diagnosed with social anxiety after her dad passed away at age 7, so what do I do in all my glorious wisdom? I ask her if she'd be my girlfriend, mind you this is the FIRST time she's seen me in real life, and out of sheer excitement in finding we share nearly identical interests I jump the gun and scare her away (so to speak).
Point is, don't get over excited, don't even try to play it cool. Be confident in the fact that you know she likes you and that you do too. The absolute worse thing that could happen is she says "no" or "I want to stay friends", you move on afterwards.
It's easier said than done I know, but you know what, you have all the power to change how you look at life and yourself. I for the longest time thought I wasn't worth two cents compared to any other guy, and I finally got fed up with it. I started working out, and changing the way I dressed (I'm Japanese/Thai an 5'3" I look about 6-7 years younger than I really am, so I'm at a major disadvantage compared to all the "suave" prince charming types who are 6'+ with faces that look like they're carved by angels). I'm way more confident than I was back in High school, it's perspective and liking who you are.
If you don't like who you are, DO SOMETHING ABOUT IT is all I can suggest to you. I'm not trying to be a prick, and I'm not trying to put you down or say you even need to change yourself. However, if there is one thing I know about most women, is that they want a strong guy, they want to know that they can depend of you if the crap hits the fan, and that starts with confidence in yourself as others have said before. She likes you because you're YOU, as long as you don't try to do anything extreme you'll be fine. If you don't ask her out because you're too afraid of "What if we break up and hate each other" you'll keep on eating yourself up with the "What if" questions. Take a risk, and you may just find your future wife or something.
So put on those Chuck Norris boots and go sweep her off her feet damn it! I have full confidence that you can do exactly that.