A Level languages are a big jump from GCSEs. I don't think I learnt anything relevant during GCSE French to be honest. All we had to do was memorise a paragraph or two and recite it to the teacher. Although I got an A* at GCSE, I don't feel like I ever learnt anything relevant in French until I studied it as an AS subject.
At A Levels, depending on your teacher, you usually go over all of the GCSE basics anyway (i.e tenses and vocab), so you probably won't be that far behind. I think the main thing for studying a language, especially if you've never studied it before, is to be a fast learner.
I'm on WJEC for French and we had to study 15 topics throughout the AS year (fashion, food, sports, tourism etc). Three of these topics come up in the writing/reading/listening exam. 16 marks of the paper go towards translating phrases from French to good English. 30 marks go towards an essay question: a 200-250 word essay on a given topic.
The oral exam is where most people struggle. Out of the 15 topics we studied, we had to randomly pick two cards with topics on them on the day of the exam, and then we were given 15 minutes to prepare for the questions on the cards, before having a 7 minute oral examination on them. 7 minutes are then improvised general conversation with the teacher.
All languages at AS are more or less set out the same way, so make sure you're ready!