Heya! Doing degree French atm and these are tips that I still use/ did use at GCSE -
1. Learn vocab throughout the year, little by little. No point learning the entire memrise set in an evening, because you won't remember many of the words. Better to learn 10 per day or 80 per week or something and actually try to use them in sentances than just clocking thorugh them!
2. Speak to native French speakers. There are apps like Tandem (where you text native speakers and they correct you whilst you correct their English) which you can use, but there are the obvious risks to that. If you have the money then I recommend Italki, and if not then perhaps your French teacher will know a French teenager looking for a pen pal! The point is that using the grammar and being corrected in the context of a conversation (or even not being corrected, tbh) is often more effective than sitting and practicing grammar exercises, since you are more likely to be paying attention and using a wider variety of structures.
3. Sadly for some, grammar is still necessary, and I found the CGP workbooks really helpful for this. The translations are particularly good, as you can mark them are work out which areas of grammar and vocab you might want to touch up on!
4. Listen to podcasts or watch tv. There's TV5Monde which is quite good, but on youtube there's Extra French which is GCSE level, and then podcasts like Coffee Break French. the more you listen, the more you will get used to picking out the words you know from the ones that you don't.