How to figure out what lens cap would fit my lens?

I have a Nikkor 18-55mm lens.

On the lens there's a designation for the diameter of the filter threads. That is also the lens cap size.

If it is this lens, the filter size is 52mm
http://www.42photo.com/pd-productid-14228-k-nikon_18_55mm_f35_56g_vr_af_s_%28_vibration_reduction_%29_lens_%2852mm%29.htm

Look inside the lens, usually behind the front glass. There should be a little number followed by "mm". That's what size lens cap you need.