Hi thats great that you want to start early but you havent covered half of the content yet as you are in year 8 i don think youve even picked your subjects yet unless your school is different but with your attitude im sure youll do well as your already planning ahead i would recommend just going through what your teacher teaches you in year 9 and 10 and start revision on the year10 summer holidays so that on september when your in year 11 its easier for you and just carry on from there good luck!!