Great choice! I studied Biology at A-level and really enjoyed it and went on to study it as an undergraduate at university.
The main things you will study include how the human body, animals and plants function. Some topics will include : Applications of Genetic Engineering, Biological Molecules and Enzymes, Cells and Organelles, Classification, DNA and the Genetic Code, Ecological Concepts, Evolution, Gas Exchange, Genetic Engineering, Genetics, Health and Disease, Homeostasis, Immunity
Making Use of the Genetic Code,Movement and Support in Animals, Nervous and Hormonal Control, Nutrition and Digestion, Photosynthesis, Reproduction, Respiration, Transport.
What you will specifically cover will depend on your school and ciriculum. If you could find out what examing board you will have you can have a look at what topics will be covered so you can start researching and reading into them early. You can also look at past exam examples to get a feel for how you will be tested.