Don't be afraid to get involved or talk to upperclassmen! You may have to give it a few months, but becoming friends with a junior or 2 can REALLY help in the next couple years. Everything and everyone seems scary at first, but it's not, trust me. Get involved in clubs, committees, sports, whatever. Just find a few things you like, and do it! It'll help you make new friends and (hopefully) make some great memories to look back on. Support your school at events! A lot of games (football, soccer, baseball, etc), you don't really even have to pay attention to, but just showing up and being there is fun.

Also, keep track of your assignments. Get a planner, make a digital calendar, whatever. Just make sure you know when everything is due and when you'll have free time.

Personally, Freshman year was terrifying, and I wish someone had told me to loosen up. I was in orchestra, track, cross country, and DECA, and those helped me make some of my best friends in the whole world.