Let's start by exploring the most common use of the crop tool... cropping, of course! Open any image and select the Crop tool. Notice in the options bar you have spaces to fill in the desired width, height and resolution for the final cropped image. To the far left of the options bar, you can select from several crop tool preset options. I'll go over the crop tool options and presets a bit later, but for now, if you see any numbers in the crop tool options, press the clear button on the options bar to remove them
There's no need to be precise when making the first crop selection, because you can edit your selection before committing to the crop. If you want exact precision however, you will want to switch to a crosshair cursor. At any time, you can toggle from standard to precise cursors by pressing the Caps Lock key. This works with the painting tools as well. Try it out. You may find that the precise cursor is hard to see in some backgrounds, but it's nice to have the option when you need it.