I am learning about signals in school but I found it very hard. Can someone help me by giving some references about any books?
We are studying Fourier transform, signal power, filters, digital and analog etc.
Without knowing what you find hard, it is difficult to recommend a specific book. Some are hard to read to. So, oldies but goldies, I suggest you to start with video lectures, and for instance by godfather A. V. Oppenheim
and tell us where you have trouble (and why).