Have you never studied chemistry or biology, before?
First tip - Do not Panic. A level year does not eat students.
Starting from Day 1; work HARD. Assuming that, you are not home schooling, I advice you revise everyday at home whatever you have been thought at school. At the end of each chapter, solve past-paper questions with out fail.
If you any doubts, post them here at SF, we are always ready to help.
Welcome to Student forums.