Have your say: Starting French A-levelWatch
I can confirm that the jump from GCSE to A level is a big one - far greater than many students realise. I often see pupils with A* in French GCSE who think they're therefore "good at languages" and who really struggle throughout the A level course. The good news is that there is a cure!
The biggest issue is verbs and tenses. Verbs are like the backbone of French: if they're shaky, you will always struggle with the language; if you know them well, you will find everything else falls into place that much more easily. If you can make sure that you can at least conjugate all the tenses (regular and the most common irregular verbs) that are on the GCSE syllabus (that doesn't mean that you'll have done them; a lot of teachers don't bother) then you'll be doing yourself a huge favour. These are the tenses you should know preferably before you embark on the A level course:
Simple tenses: Present, imperfect, future, conditional
Compound tenses: Passé composé, pluperfect.
You should also have a good idea about when to use these tenses.
It is also a good idea to practise agreements with adjectives as well as their position.
If you can, get hold of this book: http://www.amazon.co.uk/French-Gramm.../dp/095706120X. The first chapter is a really good introduction to concepts of grammar which you will need for the A level course, and it has loads of explanations and exercises for tenses, adjectives and more - it will see you through your A level course. I work with this book with my students and I find it invaluable.
Another thing you can do to help doesn't even involve French. The A level course is topic based and you need to have quite a wide general knowledge of the world around you - health, social issues, the environment, technology, moral questions are all part of the syllabus. So it helps to just read a good quality English newspaper every day.