Ah I see! In that case - take the languages as far as you can in school for sure. Definitely start exploring the texts, including in the original language if possible. There is a (old now, but apparently still good) book called "Latin Via Ovid" which serves as a language textbook but the exercises are based on excerpts from Ovid, which lets you start to get a taste of translating "real" Latin quite early on (a lot of other textbooks for learning the languages will use heavily adapted versions of key texts or just made up little stories and sentences which can get a bit boring), so that might be worth exploring. A friend of mine very highly recommended it to me.
Since classics is language based and you have to learn a lot of it via grammar, becoming more familiar with formal grammar helps a lot. This site:
http://wpwt.soton.ac.uk/notes/grammar.htm might have some helpful topics (although if you're already learning Latin you might be familiar with some or even much of it). Also make sure to keep on top of your language work - try and do some grammar drills every week if not every day, even if it's just writing out and reciting a declension or conjugation. It all helps solidify it in your mind as you practice it more and more - likewise try and work on translations as much as possible, ideally without using dictionaries (or referring to your notes and grammar tables) initially.
As above, look into summer school options; JACT is probably the best known of these. They do have some limited funding availability I think for students who might not otherwise be able to attend them. See if you can visit museums and galleries with classical sections whenever you get the chance - becoming familiar with the material culture is also important! Also explore texts in translation if the originals are beyond your language skills just now - you can access pretty much all major classical texts online free on Perseus (although some of the translations are a bit old-fashioned, they do the job!).
There are also a wealth of secondary sources which might be helpful and interesting to explore, depending on your interests. I've heard Mary Beard's SPQR recommended very often, for example. Additionally although it's maybe a bit early for it in GCSE, when you get to A-level it's worth beginning to explore the explicitly academic/critical literature to understand how people analyse classical texts academically, and some related papers on various theoretical frameworks that might be used. But for now just explore whatever is interesting to you in the realm of classics - you can start reading the theory later!