Tissues isn't that bad. Muscles isn't that bad, bones...for me a little trickier. I think you just have to have the mindset that you are going to practice, practice and practice some more and try to enjoy yourself! If you find it interesting, I guarantee that you will do better on it. I'm in AP1 now and I have a very difficult professor that has very hard exams. If his questions were worded differently I'm sure I would have an A average instead of a B right now.
Also remember, what's hard for one person may not be for the other. Oh, nerves and the nervous system is pretty tricky!
Sure doesn't hurt to get a head start. But I would start with the first 3 chapters if that's what your going to do. Bio, Chem and the intro chapter.