The issue with French is that there will always be those who're either native speakers or who've spent time in France taking the GCSE alongside those you've never left the UK. This brings the grade boundaries up, meaning that those who haven't had the opportunities that others have had will find it harder. However, there are things that you can do:
1. Immersion is key. On a day-to-day basis, you can watch french series, listen to french music, even speak to French people if you can find someone (perhaps through a teacher?). Even if you don't understand everything, practising listening especially will help tune your ear to the language and teach you what sounds right and what sounds wrong.
2. Learn how to answer the questions by doing and marking past papers/ specimen papers/ revision workbook questions.
3. Learn vocab. Learn the GCSE set vocab, and if you have time after that, start on the A level vocab.
4. Practise grammar using workbooks.