Tommy, I think you've gotten quite a lot of good advice. I'm going to build a little on just one point ... being of service to others.
Finding a way to help people could make a big difference for you with social interactions, whether it's volunteer work or a retail or customer service job. I have often found that, if I have a job to do, and if I am in some way "in charge" of doing something for someone, I have a much easier time talking, both in terms of pleasantries (how are you today, fine weather we're having, will that be cash or charge) and then, down the road, more involved conversation.
I remember years ago when I was a Saturday afternoon bartender. Business was slow at that time of day, so I got conversational practice with the few customers I had, but also had a ready exit (oh, I'm running low on ice, gotta go!) when I needed it. During that time, when I went to a party where I knew very few people and was uncomfortable, I started emptying ashtrays. Even just croaking out "excuse me, can I dump this for you?" was enough to break the ice for me.
Now, I realize you can't put yourself in *exactly* that situation at 16, but you can look for opportunities in after-school activities, at the library, your local park district, hospital/nursing home, food pantry, etc. Get some practice getting out of your own head and see if it gives you a little relief and perspective.
Good luck!
