Hey, first of all Stop and take a deep breath. I know exactly how you feel. It sounds like you’re from the US so the exams may differ slightly from here in the UK. However, for me, the way I study is to find a quiet place, fairly warm with lots of air and a big desk. Set yourself a revision timetable. Actually write it down. Plan how long you think you can concentrate for (for me it’s no more than an hour in one go) and make sure you have regular breaks (no more than 15 minutes). During your break get up, walk around and drink. Don’t over-do it, max 4 subjects a day – 2 in the morning then chill out then 2 in the afternoon.
As for how to revise, I personally make notes and answer questions from past exam papers. (I don’t know if your exam boards allow you access to past questions, but alternatively ask teachers.) Also, don’t revise into crazy hours of the night. You need sleep. You can also revise interactively online, making revision cards, reading info, covering it then seeing how much you can remember and loads of other ways.
One last thing – go out and have fun. Exercise really helps with stress. I go to the gym at my school. I also horse ride and I love reading and listening to music. It all helps
Anyway good luck and I hoped that helped.