Just in case you or anyone else looking on the forum doesn't know the curriculum, we took 7 classes first semester: Physiology (5), Biochem(3), Cell biology(2), Microbiology (2) , Histology (1), Critical Thinking (1), Genetics (1). The numbers correspond to the credit hours.
For me, the hardest classes were Physiology and Histology. Physiology was hard because it required a lot of applying what we learned and not pure memorization. Some lectures weren't as clear so I had to rely on other sources (BRS physio is a good one). I also never took a physio course in undergrad so that didn't helped. Histology was hard because all the pictures look the same to me and the figures we're looking at can be minuscule and blurred. The rest of the classes just required time and memorization.
In my personal opinion, I wouldn't really do much preparing before school starts. Just enjoy the time you have with your family and friends so you don't get burnt out during the middle of school. You'll be studying nonstop when you start so you'll have time to learn the material. I didn't prepare and I did just fine during the fall. If it's something you absolutely must do, then look over Physiology since it's the first class you start with.
Hopefully that helped. Let me know if you have anymore questions.