Stay organised. Catch-up is do-able in certain subjects, but a pain in the butt, and not something you want to be dealing with. Do your revision intelligently - don't think you can just get away with revising the content in the books. Do the extra reading, take the extra time to look something up.
DO NOT be afraid to go to your head of sixth form/college and tell them you're unhappy in your subjects. I got a month into doing psychology, decided I hated it, and told my head of year - now, I just got my 200/200 UMS in politics, despite joining a month late. It's not like being in GCSE's - you have a choice, and you need to take control if you're unhappy.
Most importantly, remember to enjoy it! You're not having to take compulsory languages, maths, or english anymore - these are all your choices. Even though it's tough at times, it's worth it in the end, and it's FAR better than GCSE's ever were.