That's amazing when you haven't started GCSE yet, wow! I'm going to reference quite a few books, but no one will be expecting you to do further reading until A level really, so just pick anything you find interesting and don't give up or assume you'll do badly if it's difficult.
One of the obvious places to start would be to read some texts in translation - literature-wise, the
Iliad,
Odyssey (note: if you want to read Homeric epic, get a character list, ideally one that includes patronymics) and
Aeneid are obvious choices, but some Greek tragedy (say
Medea or the
Oresteia) might be more accessible. History-wise, it's Livy and Tacitus. More realistically, you might enjoy some fictionalised history -
I, Claudius or the
Imperium plays. Mary Beard also writes amazing and thoughtful texts on the Classical world;
SPQR is a bit intimidating, but
Confronting the Classics is an essay collection and more fun! Eidolon (
https://eidolon.pub/) is an online Classics magazine discussing Classical texts and discourse in Classical education, which is really interesting too.
Good luck and enjoy