I have severe anxiety and depression, so I can relate to what you’re saying a lot. One thing I’ve learn, that I wish I knew when I was in school, is not setting expectations too high.
Don’t go in and expect make lots of new friends or pop un-shy. Take your time and don’t add to the pressures. Going into high school there’s already enough to worry about.
James 3:2 says “For we all stumble many times. If anyone does not stumble in a world, he is a perfect man, able to bridle also whole body.”
Don’t over think things, know that you’ll mess up your words and that’s okay, everyone does. What I’ve learned is fixating on one or two potential people. Maybe someone quiet like yourself. Look for smaller group settings like clubs, that way you’ll automatically have something in common with them and time will eventually bring you to talk more on different subjects.