With many psychological problems, half the issue actually comes from avoiding the issue. By ignoring what's becoming increasingly obvious, we bury it and allow it to keep growing and damaging us. If we accept that we've got an issue with social situations we can start to counter that issue and it's not actually that hard to do.
Start small, if you've let yourself go a bit then work on that. Dress better, improve that diet again or sort out any household chores you've left to fester. By looking better you'll start to feel better and much more confident and though you may not realise it, looking confident is half the battle to being confidence.
Put yourself into situations in which you know you'll be uncomfortable. Baby steps first and then build up to more, it gets easier as time goes on. Join your friends on a night out, go for a drink, pick up a new class after work. After a little while you'll get used to these situations and start to find it easier in all social situations because of this.
Remember not to rush it and you'll be fine. Push too hard too quickly and you could well make yourself feel worse! One step at a time and before you know it you'll be out there and having a wild old time!