The gross and nearly useless answer is you just do it. You force yourself. If you feel better talking with people, then go be with people.

I find being around people without actually talking to them sometimes is easier. Go to a busy park, or go see a movie. Yes, by yourself. At least you're not completely alone, and you're kind of experiencing something with someone else. Plus, then you'll have something to talk about - the movie, or people watching, or whatever you did.