Hey man.
I was in your shoes three years ago. There are a LOT of things I would like to suggest, but I'm only going to give you one. I'm sure you can find more suggestions on this forum. I'm pretty new to this place, but it has so much valuable content... I wish I would have read this going into my freshman year.
Anyhow, High school is going to be different. The best thing you can do is to approach it with an open mind, and seize every opportunity to get to know people.
There are many great people you will meet throughout high school, but you may never meet them if you don't have an open mind and make an effort to meet them. Think about your best friends now. Think about how you guys met. Every friendship/relationship starts as a simple hello or a similar interation. So seize every opportunity to start one.
Let me know how things go. Best of Luck.
Watchin your back,
Big Brother.