The poster that said you have a bad view of yourself hit it on the head. If you don't like yourself, how can you expect someone else to like you?
I don't know how women go about improving themselves, but when I was younger a lot of the girls I was interested weren't remotely interested in me because I just didn't have anything interesting in my life going on. I was good for a boring conversation, but when it came to hanging out or partying, I didn't have any suggestions or experience, so they would seek other guys that did.
I say focus on making yourself more interesting. Whatever you like, completely immerse yourself into it. If you like a certain band, see if you can go see a concert. Go by yourself if you have to who cares. Get some stuff to talk about and experience with having fun. Once you have intersting things going on in your life, you'll be like a magnet, because you'll have fun stuff to do and talk about.
Good luck and don't be so down on yourself. It's highly unattractive. If you have to go walk through a mall in your best outfit with yoru sexiest walk and tell yourself "I'm the sexiest woman here and all the guys here want me." You'll notice by just that change of mind, guys will sneak glances at you because you will be carrying yourself with more confidence.