Our head teacher always said in assemblies that GCSEs reward hard work and patience and I would tend to agree with him: they're time consuming more than intellectually channelling.
The place where I nearly fell down was on technology coursework. I think I've said this in another thread but if you're doing a technology or something else that requires extended coursework, stay on top of it, do not leave anything too late and allow extra time at the end for unexpected delays.
Don't worry about GCSEs now. When I was in your position, I was really scared too, but they're not that bad.
Other than this, Magistl's advice is pretty sound.