File Prep

I always seem to stumble upon much confusion when dealing with colour and page size during printing processes. Although I know what RGB and CMYK are, every monitor screen and printer seems to differ. It can be quite unpredictable and therefore I would like to broaden my knowledge in this area. Similarly, I constantly manage to make mistakes around size of a page when printing. Something must go wrong in reference to crop, bleed and registration marks because I am either cut off or there is too much white space. I definitely need to take more caution when prepping my file.