Photography Gnd

What Filter? Multiple-Part Questions.?
I was reading quite a bit on filters in Digital Photography, and I've picked out both High-Resolution UV Filter and a High-Resolution Polarized Filter. I've read on what they've done and decided that I would want those as well as a Graduated Neutral Density Filter. My questions are: How do I apply them (On the attachable lens or on the body's lens), what size would I need (Depends on the first question), and I can't quite find a GND filter, would you know of a good place I could find one?
My Camera is a Nikon D90 with a Nikkor 18-55mm DX lens and a Nikkor 10-24mm DX lens.
The filters screw into the threads on the end of the lens. Your lens will state the filter size required. You will see what looks like a 0 with a line through it followed by a number. That number is the filter size you need. Each of your lenses may require a different size filter. If this is the case, you can save lots of money by only buying the largest size filter, then buying a step up adaptor that will allow you to use the larger filter on the smaller filter size lens.
Here are plenty of GND filters. Just pick the size(s) you need:
http://www.bhphotovideo.com/c/buy/Graduated-Neutral-Density/ci/15073/N/4294271538
steve
Graduated Neutral Density (GND) Glass Filters Tutorial (HD)