First off, don't work yourself too hard! Enjoy the summer between year 11 and 12 because the one between 12 and 13 is going to be nowhere near as relaxing.
One thing you can do to prepare is if you know your subjects and exam boards then look up their specifications to see what you'll cover over the course. If you want to, you can get copies of the revision guides or textbooks and skin through them. Personally I just used copies from my college library and didn't actually buy them myself which I'd recommend but each to their own.
Make sure you've got everything you need ready too! Pens, pencils, ruler, highlighters, calculator (if applicable), a big folder for each subject and a way of taking notes whether this be one A4 pad for all of subjects or one each or a laptop, etc. Being organised from the start is the best option!
To be honest unless your sixth form gives you summer work, there's not too much you can do to prepare so just enjoy yourself! If you have any questions please feel free to ask.