Well, I'm in year 11 too, and I'm not going to brag, but I'm doing pretty well in chemistry.
Man, if you're not doing well in chemistry, you're doing more than a few things wrong. You really have to put the effort in, in order to be on top of your chemistry work.
Another thing that shocks me is the fact that you dont understand the book....you're obviously not paying attention.
Do you pay attention to the teacher? Do you write notes when the teacher is talking? Do you do your homework? Do you write down notes on the syllabus?
Do these and you'll be on top of your work. There's no easy way out.
This is stage 6.