When should I start revising for GCSEs and what is the best strategy for revision?

You should start you revision as soon as possible so that you have the time to put the different ideas, facts and names in order and form stable relationships between names and theories! You could even start planning your revision while you are still learning the material as then you will have a good idea of how much time you need to put into something (for example if you find something harder to get your head around). In term of strategies, making a revision plan with deadlines will help you to focus and keep progressing on the revision. What is more, a good idea would be to make a to do plan each day that will motivate you to not only start but also proceed on the tasks at hand.