I have 18-55mm lens on my nikon d3100 what size filter would i get?

Most places only sell 52 or 58 mm filters

The 18-55mm refers to the focal length of the lens.

A filter is based (roughly) on the diameter of the lens.

If this is your lens: AF-S DX NIKKOR 18-55mm f/3.5-5.6G VR

then you need a 52mm filter. If not, take the lens in with you to a store.

One big piece of advice. Do NOT buy filters that are inexpensive. Pay extra & get good, high quality filters such as Nikon's.eBay often carries them. The reason? A filter affects the quality of your pictures as much as the lens. Nikon makes good lenses, don't put a piece of crap in front of it.

You'd think that turning over the lens cap was common sense and knowledge by now.

Ah well, 52mm indeed.

You have to use the 52mm filter no other will fit.

This works for most lenses

Lift your camera.
Look at the front of the lens
You will see writing around the glass of the lens including a symbol like this Ø, followed by a number, for example 55. = Ø55
This is the size of filter you will need.