I'm 17, and never had a girlfriend before..?

It just hit me, and now I'm really unbelievably depressed about it. I don't really have a problem with talking to girls, but when it comes to talking approaching them, it seems nearly impossible for me. There was this really cute girl that I saw in the hallways when I was still at school, and I think she might have actually liked me back. She glanced over at me a lot, she's smiled a few times, and one time when I turned to my side to look at her, after I looked away, I heard giggling coming from her direction. I've had a crush on her for like 8 months, I've never liked a girl this much before. I never had any classes with her, so approaching her would be a matter of me going up to her in the hallway, but just thinking about that makes my heart race. I ended up never approaching her, and I feel like **** now. On the last day of school, I mustered up the courage to actually smile at her, but I was in a huge crowd of seniors (we were waiting to get our cap and gown), but I don't think she saw me. Well now, I added her on facebook (she was tagged in one of my friends pictures) and I'm thinking about facebook chatting with her, but I get so nervous, even on facebook. I don't know why; I'm pretty sure she likes me, but I always have negative thoughts lingering in my head. I always think "what if she thinks I'm a creeper?" but at the same time, I'm pretty sure she likes me, but it's like negative thoughts just overwhelm the positive ones. I don't really think I'm bad looking http://a7.sphotos.ak.fbcdn.net/hphotos-ak-ash4/208850_212749118741769_100000200473356_955694_1714606_n.jpg The crazy thing though, is that when I talk to girls that I'm not interested in, even if they are attractive, it doesn't seem that hard, it's like I'm just talking to any other person. Yeah, I'll be a bit shy if I don't know them too well, but I won't get super nervous like I do with that one girl. I've had like 2 girls tell me I'm the funniest guy they've ever met, which really REALLY put my self esteem up, but I still feel nervous about talking to her. What should I do? Should I just try talking to her on facebook? Girls, if a guy you found attractive started talking to you on facebook, what would you think?