When I started high school and left my friends behind, for the first couple of days I became a more outgoing version of myself. I said hello to anyone I didn’t know and basically tried to get the feel for who I wanted to be friends with. Luckily I found a few nice people so that I could go back to being my quiet self, but being loud for the first few days would be my advice. At best, you’ll find some great friends. At worst, you’ll know who to avoid later.
My other advice would be to ask lots of questions. Ask upperclassmen (or anyone, if you’re transferring in) about what teachers to avoid, what their classes are, or how to get places. Most people are happy to show off what they know, or if they aren’t, again, you’ll know who to avoid.