There are two parts to focus on for maximising your grade for GCSEs (particularly for Sciences), which is knowledge and exam technique.
For the knowledge part, make sure you go through the specification and have covered each point in enough detail. Make flash cards so that you can practice 'active learning' rather than 'passive learning', where you simply read information passively. Mind maps are another great way to jot down what you know about a subject.
For exam technique, practice makes perfect! Make sure you know how to answer questions, using terminology that examiners like to see. Learn how to structure your answers in ways that make logical sense, which make it easier for the examiners to mark.