Being a guy, means to be rejected. Yep, it's the cold truth. Accept it now. What you need to focus on actually is turning the experience of being rejected into a positive one. A positive outcome is you will feel confident that at least you tried. Nothing is worse than regretting you didn't give it a shot, because you never know what could of been. Another thing is learning from mistakes. Pay attention to what you are doing when trying to ask a girl out. Did you look her in the eye? Did you stutter? Were you slouching? Being rejected allows you to identify your pantomime mistakes, and fix them next time.
Don't ever think that a girl (or a person) is too good for you. Don't place girls on a pedestal, they're people with individual needs, wants, desires, fears, etc. Just like you. To reduce the chance of being rejected, by getting to know a girl a little bit before hand. Ask her about hobbies, interests, etc. See if she jives with you. There's a possibility you'll find out the girl you like is really some self-absorbed dullard. Then you just dodged a bullet.
Finally, be yourself. Don't ever change yourself for anyone, no matter if it is to appease some pretty lass. Now this does not mean to never grow up and never mature. It means if you're a guy who likes movies but not art museums, don't be afraid to state this. You can keep an open-mind, but just be honest with her and yourself.
I have great faith in fools; self confidence my friends call it.
- Edgar Allan Poe