A great resource for a somewhat confusing topic for photographers, X-rite have release a series of videos on colour management. If you are unsure what a colour managed work flow is or have some questions hopefully they will be answered here. Of course they are plugging their brand X-Rite, but the same principles apply with any calibration tool. The first and absolutely most important step is to calibrate your monitor. Check out the videos here: Colour Calibration, How to
And no, MACs are not exempt from Colour calibration!