I have the same issue, except it's probably worse in my case than yours. . .I have mild depression/obsessive compulsive disorder, and on top of the whole antisocial/lack of self-esteem thing, mild paranoia tends to settle in once in a while. Yeah, I got problems.
First off, you want to figure out where your antisocial behavior stems from. Mine comes from the fact that I'm absolutely TERRIFIED that people will just reject me if I make some kind of social related mistake (i.e. stutter, mix up words, say the wrong thing, whatever). I don't know if you have the same issue, but usually knowing what the problem is helps.
Next, spend more time with people who share similar interests with you, and HANG ON TO THAT SLIM LINK LIKE GRIM DEATH. For myself, I used to NEVER talk with people, ever. . .I'd respond with monosyllabic grunts. After I started Smashing competitively and seeking people out for better competition, I found that I could relate to these people (sort of), and strike up a decent conversation with them (sort of; it tended to be Smash-related, of course).
In time, I realized 2 things: first that having a common interest with these guys made social communication much easier, and secondly (and most important) they weren't as judgmental and prone to rejecting others as I feared they would be. In fact, I realized that the fears I had in my mind were completely blown out of proportion.
Eventually, I picked up more interests (namely Photoshopping, guitar, and working out at the gym), and that helped me to relate to a lot more people. Conversations that were once limited to those specific interests eventually expanded, so that communication--and eventually friendship--became possible. Heck, one of my best friends is somone I met only a year ago, but we became friends mostly because I bugged him about Photoshop stuff and guitar.
That's what happened to me. And I'm not completely cured, but I'm a hell of a lot more social than what I was before. So basically, in a nutshell:
1) Be proactive about seeking out other people, no matter how terrifying it may be.
2) Antisocial behavior usually stems from fear of rejection. . .if that's the case with you, realize that the fear in your mind is exaggerated and unrealistic.
3) Choose an area of interest, and seek other people out with similar interests--it'll give you the social experience you need and the interaction will build your confidence.
4) Start getting involved in more things so you can relate to more people in different areas--art, music, sports, anime/manga, videogames, whatever; you'll meet and interact with more people.
5) At this point, you should be a lot better off than you were at step 1). You should now find a girl and get laid.
Okay, I was joking about the last part of 5). But you get the point. Hopefully all that should help you a little. I know that it sort of worked for me. The problems are still there, of course. . .but working through the above stuff should get you on the right track.
Plus, a tip: if you want to keep a conversation going, just repeat the last thing that a person said in the form of a question. Example:
Talkative Person: So yeah, I saw
300 yesterday, and the visuals were awesome.
Toecutter: The visuals were awesome?
Talkative Person: Yep. It's all CG stuff, and they made these really weird brown/blue tones. It looks just like the grapic novel.
Toecutter: The graphic novel?
Talkative Person: Yeah.
300 is actually based on a graphic novel by Frank Miller. He's the guy responsible for the Batman graphic novel
The Dark Knight Returns.
. . .like so. Obviously, you can't use this trick too often, because people will start to notice. But it'll help you out of some tight spots.
