Well, you are still very young, so a lot of your habits can be changed relatively easily. I used to hate talking to people, especially public speaking. When I first started college at around your age, I was pretty crappy when it came to you know, being nice to people, talking about anything and not sounding like a babbling moron, and basically anything that required social skills.
I would just say, start out slow. You need to get that practice, it is absolutely essential. It is very difficult and I won't pretend that it isn't, especially since we tend to have the wonderful baggage of PTSD. I wouldn't go to a crowded area at first, maybe like, find a friend (or someone approachable) from your class and ask him/her if she wants to get lunch, a snack, or coffee and just try to do a little small talk. Look for opportunities to ask the person questions and make a game of getting to know them better. You don't have to keep hanging out with this person or even become best buddies with them, but doing an outing every once in a while, going to a small study group when you have the chance, etc will really prove to be great learning lessons. If you do start to have a panic attack, just tell them you have a heart condition or something along those lines. They will understand that your reaction is out of your control and you don't have to reveal to the world that you have PTSD.
Much luck to you :)