Hi there. Wow, you sound so much like a younger me! I took my GCSE's and got 6 A's, 3B's. I was also so, so worried, but having gone through it all, let me give you some advice which I hope you will find helpful.
1) if your teacher ever tells you to revise for an end of unit test, just do it. I used to ignore my teacher and not revise but looking back I really wish I had because there is so much to remember.
2) On the day of the exam, make a rule with those that you are standing with that every time you want to say something like 'IM GOING TO FAIL', instead try and say something that will make it all seem better- there is no point getting worked up because if anything it will make matters worse.
3) Re reading and highlight does NOT work- just don't do it. Of course re read, but alongside that make flash cards, posters etc. The website Brainscape and Memrise will be your new best friends.
4) GCSE's are not the end of the world!! Believe me, there are so many opportunities out there and your grades will not define you. Do not compare yourself to others and be proud of what you achieve!
5) If you suspects that you might need tutoring for a subject, start getting it ASAP
6) VERY IMPORTANT. When revising, DO NOT do everything in one go. I study psychology and trust me, cramming it all in just doesn't work. My advice is to work in 15-30 minuet chunks, and then at the end of that when you have finished, take a 5-10 min gap and TEST YOURSELF. Testing yourself is the absolute best way of revising. If there are any particular subjects you are worried about, please feel free to tell me as I might have taken them and might be able to give some specific advice.
7) If you are looking into going to college, look up the college you are thinking of attending, type in the 3-4 A levels or Btech's or anything else that you want to take and see what grades you need to get into your course- that way it gives you something to work towards.
8) Your maths and English are now your most important subjects. These are the key for getting into College's and are what people will look for. Don't pass either of them? Dont panic! You can take them again at colleges and you will still get into the college, but you will have to re take the GCSE's alongside your other courses
And finally
Breathe. You are still in Year 9, you have work experience (etc) to look forward to yet! As long as you are doing regular revision with flash cards, self testing and mindmaps you'll do amazingly! Like I have previously said, if there are any particular subjects you're worried about, please feel free to reply and tell me. Sure, GCSE's are scary and are your first proper exams but your personality and traits as a person are far more important. X