First, it's important to remember that you're still in year 11, so even if you want to do great in your prelims but something goes wrong, nothing will count. So don't stress.
Essentially, what you should be doing is finding out how much time you have to study for your prelims and divide your time accordingly. It would be a good idea to list your other subjects here so people can provide more specific advice for each subject. But like I said, dividing your study time into tasks to do for each subject is a very good start to your preparations for your prelims.