All the topics that you need in GCSE biology are cover in the BBC website work your way through them and do the little quiz at the end of each topic- this is one sure way of doing well in the exams.
There are also a number of exams you can work on as well and these can be found at the links below:
Sample paper 1:https://filestore.aqa.org.uk/resources/biology/AQA-84611F-SQP.PDF
Sample paper 2:https://filestore.aqa.org.uk/resources/biology/AQA-84612F-SQP.PDF
Paper 1 2018: https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2018/june/AQA-84612F-QP-JUN18.PDF
Paper2 2018: https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2018/june/AQA-84612F-QP-JUN18.PDF