One thing I can tell you is don't slack off on 1 or 2 subjects, no matter how stupid it may seem. You might think that say your Art or Music GCSE is pointless, but when it comes to sixth form/college and uni if you have loads of A* and A grades and then a random C or D, they may question your diversity and dedication.
As long as you work hard, in the correct proportion to resting well, you can succeed.
Also, don't lose sleep over work or things like that. Even though you should try your utmost hardest to avoid it, sometimes a piece of coursework may be a bit late because of your revision for another subject. Don't worry to much, GCSE's are a time when many people do develop mild anxiety and I know I even developed insomnia. When you think about it, it's not worth it is it? That extra day or couple of hours of work will not matter all too much, so remember to take a breather and clear your mind because your health and well being is always most important. If you need to, talk to your form teacher or head of year, they always know how to help - they teach and talk to so many students every single year that have probably gone through the same thing.
All in all, good luck and enjoy your time at school! It is an experience as well as a good education.