What sized UV filter for my camera?

I'm new to photography and I have a Nikon D40 with an 18-55mm lens. What sized UV lens do I get to fit this so I don't scratch the lens? Do I get a 55mm filter to match the highest number… Orrr?

It should say the diameter on the lens. 18-55mm is the focal length not diameter of the lens.

Your lens uses 52mm diameter filters. Choose your filters from B+W, Hoya, Heliopan or Tiffen. Always remember this: "A cheap filter isn't good and a good filter isn't cheap." A good on-line site to buy from is B&H - http://www.bhphotovideo.com/...ovideo.com

Or you look under the cap. I'm guessing it says 52mm. That's the diameter of the filter thread.

The Nikon 18-55 kit lens has a diameter of 52mm, and this is the filter size you should be looking for.
The 55 is just the highest focal length and it has nothing to do with the filter size.
Cheap filters are not so good, they just protect the front glass being damaged, kind of. But they may cause a little bad effects on the photo such as flare, lower contrast, etc

Don't waste your money on a UV filter. A lens cap protects the lens. Get a filter you can actually use like a polarizing filter. Always buy good filters.