heyy, i am also taking German for GCSE. I think the best way you can revise, is to download a few German apps (my personal faves are German, Memrise and Anki). In Memrise and Anki you can create your own German flashcards. I also like to watch my favourite tv shows in German and try to have conversations with my friend in German as well. In German you basically only need to learn your vocabulary and word order, so I think it is best to do that first.