Aqa gcse citizenship studies helpWatch
I was on Edexcel so it might be slightly different, but make sure you practise some of the longer questions (12 markers in the case of edexcel). Also, learn your key terms because there is often questions which approximately say "define (say) first past the post" and if you don't know what that is then you've lost the mark which is a bit annoying (did exactly that on my mock, and it cost me the grade).
But really, citizenship is pretty simple. I did mine in 2015, when I was in year 9, did one past paper as revision and got an A* on the paper, A overall. The election was the week before so I had been paying pretty close attention to that (because I found it interesting rather than for revision though) and I put a lot of specific knowledge from that into my 12 marker which I'm sure boosted me up significantly so try and concentrate on one section in particular, getting extended knowledge about it to use in whatever longer question comes up. (I'm assuming you have one lol).
Past papers are a really great help so use them to get used to the exam style and answering questions within the time period.
Other than that, relax! A citizenship mock is most definitely not going to change any of your future plans, so don't worry about it, it's just an extra free GCSE you can have