that's a pretty broad question. Is there something broken you are trying to fix or you just want to learn about computer hardware in general?
If your goal is to learn about computer repair, I find this book to be very valuable in your toolbox. This is the book I use to teach from (just the older editions since its been years...) back in the days of teaching A+/computer repair. I dont think there is a better reference book. Just watching video tutorials is fine for learning the steps, but you really need to have a solid understanding the hardware components themselves... if what your goal is to learn about computer repair.
Thanks jorgem , "upgranding & pc repairining " is very good book for those who want to learn hardware from begning , if an one want to learn advanced level repairing then this site has a great article about voltages flow in whole motherboard , remember if u learn only voltages flow than know mater which kind of mother in front of you for repairing.