Don't start too early as you'll have lost all motivation and be extremely fed up of revision by the time it really matters. If you really want to start early, just go over topics/subjects that you're not as confident with, or stuff you feel you are particularly behind on. Honestly, depending on who you are, you could start anywhere between 2 months or 2 weeks before the first exam and still do great! Makesure you take your mocks and other exams seriously as they'll give you accurate predictions on where you're working at, which you can use to find where you can improve and what you need extra time on. Don't be afraid to ask your teacher for help after class if you find a topic which your struggling with. (Imagine how annoying it would be if that topic came up a lot on your exam but you didn't bother to ask!) Also, with mock exams and other tests you do in class, experiment with different revision techniques to see which ones work best for you. For some people, cramming can be extremely effective, whereas for others it is useless. Try and have an idea of how much time you'll have to spend on each subject to get the grade you're aiming for - so when you're revising for mock exams, do you take simply a day of revision for the exam or more like a week? Both is completely fine, it just depends on who you are. You'll probably be advised to make a revision timetable, but I personally found this ineffective and extremely hard to stick to. As said before, focus on your coursework! Don't act like it doesn't matter and you can just do well in the exam, because you never know what will happen. Getting the highest mark possible in your coursework will not only help towards your grade, but will also make you more motivated as you will feel like you can get the grade you're aiming for. Try not to stress out during the exam season as it will only make matters worst. If you're feeling particularly stressed, try and take a break to do something you enjoy. Take short 5 minute breaks during revision also as this will prevent you from getting too bored and overworked. Also, talking to your friends about their revision and how they're doing will be a great help!
Oh one last thing, turn off your phone during revision. It is only a distraction and will make it even more difficult to concentrate.