There is a list of topics covered by the MCAT on the AAMC website. You could also pickup review books (Kaplan, PR) and note the things that they cover. Generally the review books will include every detail you will need to do well on the exam, so I wouldn't focus on material that isn't in the review books.
However if you're just trying to keep it for your general knowledge, obviously you can study whatever you want. Keep the notes (digitize them if they're on paper) so you have them for future reference.