I've been an SAT/ACT tutor for about 23 years. Don't be afraid! You should arrive early enough to get comfortable (about 7:45 most likely). There will probably be someone at the entrance telling students where to go because there may be students from several high schools (yes, mostly 11th and 12th graders) taking the test there. You will be sitting at a desk alone but in a room with many other test-takers. The proctor (teacher in charge of the test) will probably hand you a test booklet and a test answer booklet.
Read my blog for advice about what to bring with you, but basically you'll need photo ID (school ID or letter from your school identifying you), #2 pencils (several with good erasers), a calculator, some energy snacks, something to drink for the break, and a watch. You won't be allowed to have your cell phone with you.
Check out my blog and you'll find other info about what to bring, what to eat, what to wear -- that should make you more comfortable.