Hi,
I've don't do a-level biology or chemistry but I do study history.
I would say the main thing is to keep on top of your notes/flashcards/whatever you use to revise because there is a LOT of content and teachers tend to move quickly through the course. In terms of essays, I'd say just to practise - don't worry if your essays aren't as good as you want them to be, they'll slowly get better over time if you try to apply any feedback you get. Try to practise your essays in timed conditions like you would in the exam, just so you get used to how quickly you need to write. But to be honest, at the start of year 12 mainly just focus on understanding and throwing yourself into the content. Hope this helps!