usually most will buy UV filter which also acts as a protection filter to your precious lens.
CPL is often used in landscape photography but since you are very new, you need not buy this filter and explore it till you are comfortable with your camera.
For UV filter, I will recommend you to get Hoya Pro1 digital series.
Just rem, just dun get a cheap (hint: tokina) UV filter that gives you ugly flares to your photos.
Nikon D90 | AF-S 18-105mm | AF 50mm f/1.8D | Tokina 12-24mm f4 | Sigma 30mm f1.4
Maybe this link will give you more info on lens filter: HERE.
Current: 550D + EFS 18-135mm IS + EF 50mm f1.8 II