Also, you want to buy the highest quality you can afford. Poor CPL ruins your photo very badly.
In my personal experiences, CPL always get one corner unevenly darker. So, I have stopped using CPL. In most of the cases, I use a ND 4 instead to increase the contrast.
If you have to buy one, buy the biggest size of all your lenses. For Nikon, I think the most useful one is 77mm.