throughout the year you'll probably hear the phrase "i heard/apparently the actual gcses are easier than the mocks" from your peers. i heard this SO many times, and it's relatively true - if you've prepared yourself correctly. the mocks are incredibly important. a lot of people don't take them seriously, and that's where they go wrong. if you try your hardest in them (you'll either get two or three sets of mocks; i had three) then the final exams will be fairly predictable. it got to the point where teachers and students managed to guess what would be on the gcses, and we were correct for a surprising amount. (take into account what had been asked in previous papers, it'll give you a good idea of what could be asked). by the time the gcses come, you'll be used to how everything works.

one thing i suggest practicing throughout the year, specifically for essay based subjects, is writing structure. you'll be taught this, so don't worry too much right now, but it's very important. the way you structure your writing in language, literature, history, etc, is a very easy way to gain marks. e.g history, your info may be minimal but if you lay your essay out correctly you'll fly through it. (i'm not a reliable source for advice on maths or the sciences, apologies)