As above, but there are definitely phrases that you can learn and learn how to use in a variety of circumstances. Generally, however, you should just try to strengthen your grammar to the point where you can use it comfortably. That being said, there are tenses/ moods such as the subjunctive and the passive which you may not focus on much at GCSE!
- Bien qu'il y ait... (although there is/are) = this is the subjunctive form of 'il y a'. You could say something along the lines of 'although there's a cinema, I'm bored.
- En discutant... (When discussing...) = I'd use this to start a phrase. You could say something like 'When discussing the climate, one must mention...' but you could also say 'When speaking to my mum, I learnt that...'.
- Je suis en train de... (I'm in the process of) = nice idiom to use to describe something you're currently doing. 'Je suis en train de lire mon livre'.
I also recommend getting to know the passive mood. It isn't too tricky, but it's worth knowing!