In biology it'll take a while to be getting B's+
After a while (hopefully not too late) you'll understand how to answer stuff.
Just picture a mark scheme and think, "hmm, what will the examiner want to see"?
For chemistry, if you know the theory, the only other thing left to learn is the definitions and equations
Obviously do past papers and mark them. Thats the key btw. at GCSE you'd just do questions and not care about the actually answers. at A lvl u NEED to mark everything u do so u learn