Hi,
I have a nikon FM10 and I want to buy a polarizing filter.. which one should I use?

I like Hoya for filters. What lens do you have on the camera?

My camera has the kit lens which is the 35-70mm lens...

You should see a number in millimeters (mm) around the front edge of the lens. This is the size you need. From there, try froogle (http://froogle.google.com/frghp?hl=en&tab=wf)and ebay (http://www.ebay.com)and search for

nikon polarizer OR polarizing XXmm (of course replacing the XX with the mm designation on your lens).

Generally I've found accessories like this cheaper online than in local camera shops.