I can understand what you mean perfectly- I'm 21, never done ANYTHING non-platonic with a girl, and generally back away from talking to girls I find attractive. And due to my being a fucking moron sometimes, I drove away a friend (who i already had no chance with, the saga of which is in another thread) who I was interested in. That's why I'm trying to change.
You say you're not thin - start working out. Going from 235 lbs to 175 lbs over the last 16 months (still have maybe 30-40 to go) has already made a difference- girls look at me differently (this from reliable sources- and note that differently doesn't mean interested, but it's still more positive than before), they seem to have less of a problem being near me, and I actually feel a bit more confident- and it's all around better for your health to get down to a decent weight.
Try to talk to people more. Smile more. Learn to strike up brief conversations with random people. I'm no expert at any of those things, but from what people tell me, it's important to have those skills. I have no intention of becoming a 40 year old virgin who's never dated, and you shouldn't either. Get out there and change. And ask any friends you have who are the complete opposite of you to help. Good luck.