You'll be told this by everybody, and you probably won't believe it, but YEAR 11 GOES UNBELIEVABLY FAST! For starters, you're finishing about a month before you're used to, anyway, so the year is already a month shorter than you expect. Your exams probably start about a month prior to that, so that's two months shorter than you're used to. The point is, you have just over eight months from the start of Y11 in September until your GCSE exams start.
Take the mock exams as if they were real. Try your absolute best, because that gives you a confidence boost for the real exams; if you got <insert grade here> in your mocks four months before your real exams, you are quite likely to do better in the real thing!
Make sure you understand everything, particularly in subjects such as Maths, as you learn it. Go to teachers at lunchtimes and stick to them like a limpet until you 'get it' because IT WILL STICK! Then when you come to doing your revision you will already be familiar with everything you need to know, thereby preventing any last-minute scrambles to learn the entire specification.
When February comes around, start reading through your notes and revision guides. This will refresh your memory on important concepts that you need to know, ready for you to start revising fully in March/April. Don't get too hung up on exactly when you start revising or how much revision you are doing; everybody learns at a different rate, so just make sure you leave yourself a good amount of time to avoid stressful cramming. Your mocks are very useful here: ask yourself honestly how much you revised, and how that was reflected in the results you got. Did you get A/A* grades across the board? Great - your strategy worked! Did you slip below your targets? Start a bit earlier, this time.
Remember that until you have done the paper, nothing is set in stone. Your results depend on these next exams. To an examiner, your mocks are irrelevant. This is good, because it means that you can improve, but also bad because you can let yourself slip: DON'T BE THAT PERSON WHO BECAME OVER-CONFIDENT AND DIDN'T REVISE ENOUGH; IT WILL BE REFLECTED IN YOUR GRADES!
Over the course of Year 11, I found four things which I found particularly useful. If I could only give four pieces of advice, they would be the following:
1. Immerse yourself in the subjects. Read around them. Find things beyond the specification, and truly become knowledgeable about your subjects. There is a distinct difference between information and knowledge: the former is simply facts and figures in isolation; the latter links them all together. By truly knowing your 'stuff', you will find it much easier to write coherent and generally 'good' answers to exam questions.
2. Spend time thinking critically about what you are learning. Take nothing for granted. I'm not saying that you should spend the whole year deciding whether 1+1 really is equal to 2, but that you should try to think beyond the confines of the specification. This links with point 1, because it helps you to build up knowledge rather than just isolated pieces of information, and by virtue of the time you spend thinking about things you will find that you retain that knowledge much better.
3. Look up things which you don't know, and apply them. If you come across an unfamiliar word, look it up and use it. If you come across a concept which you don't understand, research it. By doing this, particularly if you do it quickly, you quickly build up that all important knowledge and understanding.
4. Mark all of your past papers. Marking any past papers you do is almost as important as doing them in the first place. You will quickly develop a sense for what a question is asking for, and this will help you to hit marking points in the real exams. I would suggest marking a paper on a separate piece of paper, recording what you give yourself for each question, and writing down what you needed to get anything you missed. You can then give the paper to your teacher to mark, and compare the marks you both gave. How similar was your marking to your teacher's?
Most importantly, remember that mitochondria is the powerhouse of a cell!
Good Luck!