Chemistry - spend your time learning to understand topics, don't just memorise a sequence of events of a reaction etc. Some aspects (definitions) will require that, but at least the summer module is far easier if you learn why things are happening, because you can apply it to any other unknown situation then. The trends and patterns you'll learn can logically be remembered by using the periodic table, if you understand what parameters affect what things. It's a pretty logical subject, once you get into the right mindset.
Biology - far more about rote learning things. Just keep on top of it. Condense your notes into bullet points and break down sections. The revision guides that exam boards sell are usually quite decent to help you with that.
Pay attention in class, don't be afraid to ask questions, and use your free periods wisely. Sit with your friends for a couple of hours a week maybe, but perhaps think about going to the library and revising what you've done on a module so far before a lesson etc. Or, do your homework in them if you know you won't concentrate on it at home.