I had a good background in Biomechanics. One thing I did for my classmates was complete the book problems and show step by step how I did my work. Then they would practice a similar problem using my work as example. I also tutored students in groups. We would meet in a room and put everyone up on the board to do the same problem at the same time. We worked through it step by step, and then did another similar problem with less help.
I think it is the best to use the book/online material your school provides you with since you probably will be tested on that specific material. Outside resources may be better and easier to understand but they may not prepare you to your exams. I would try to spend more time to slowly read/re-read the material your school provides you with and try again to understand it or maybe ask your classmates to clarify things that are too confusing for you.
