Like Stan Rogers points outStan Rogers points out, all that you need to know is the filter diameter for the lens, which should be listed in just about every data sheet or review of the lens that you can find. The AF-S DX NIKKOR 18-55mm f/3.5-5.6G VR (which I take it you mean) does indeed take a 52 mm filter according to Nikon's web site and a quick Google search.
However, you don't need to buy an original lens cap unless you specifically want to for some reason. There is no functional difference between an original lens cap and one of another brand. Picking up a no-name cap of the same diameter is most likely to get you exactly the same results for half the price or less.