How and when would I use the subjunctive mood?

The subjunctive mood is a tricky one, it's usually used to express something that might happen, or when there is doubt that something will happen -  so it's commonly used in wishes or commands. It's also used with a few set phrases such as "Il faut". At A-level, it's unlikely that the subjunctive will come up very often on exam papers, but it is a good way to impress an oral examiner or gain some extra marks when you're writing an essay.
     The best way to use the subjunctive at A-level is to learn off a set phrase and try to use it when it fits best in these situations. One good example is "Il faut qu'on fasse..." which means "it is necessary that (one does/makes)...". You can pretty much slot this in any scenario when speaking or writing in French. It's also a good idea to have a glance over the subjunctive mood in some verb tables for common verbs like to be, to have, to do/make - that way you can recognise how the subjunctive might look when it's written down, even if you don't necessarily have to use it.