Well, if you are a hikikomori, i can only recommend gradual and increasing exposure to the outdoors, individuals and crowds. Maybe go to the park, relax on the bench in the shade for awhile? Then scale it up to strolling around the town, or something.
Another thing might be to start learning a foreign language, and when you have a decent grasp of it, go to that country and travel around for a month or so. I've heard that sort of thing does wonders for confidence, and you certainly don't have to worry about any embarrassments, as you will never see those people again. Although this may be a bit much.