The best medicine for this sort of thing is to force yourself. Get a job where you have to interact with people. I don't mean a grocery clerk were its just "Hi, that will be 10 dollars, have a nice day" crap. A job where you rely on coworkers. There are other ways, but this is easy, and you can get paid in the meantime, so even if you have a shitty day its not a total loss. Alternatively, you could try for trade school or adult education and get a GED. Nice benefit there is that if you have a rough time of it, you might have some kind of advisor/counselor you can talk to.

In the meantime, work on your self-esteem. Start exercising and improve your diet, and get a hobby, even if its a nerdy one.

Really... inaction is the only way you can fail here. I know this first-hand.

There are a lot of people out there who suck, and there will be lots of people out there who think you suck. The only way to find out if you can make friends is to sample the pool, so to speak.

That'd be what I do, anyway. Admittedly, my own social life is terribly modest, I pretty much only interact with coworkers... I've been invited to things like golf outings, but I really don't play golf and the like. My friends are all pretty much gone now, so I spend a lot of nights playing video games. In short, I feel you.