For Chemistry, firstly, I highly suggest getting into a routine of regular revision since it is a very content heavy subject (my school decided to do some yr13 topics in yr12 as well). As per revision resources:
Youtube: AlleryChemistry, The Organic Chemistry tutor.
Websites: Chemrevise, DocBrown's Chemistry (great resource with literally everything and not many ppl know about it idk how, it was a bit difficult to find, but its great)
Also if you have great teachers, please use them first if you have any doubts.