You ask about an 18mm lens; on a DX format sensor, 18mm is not very wide, having the approximate equivalent angle of view of a 28mm lens as an FX format (film sized) sensor. In that case, the "kit" lens of 18-55 would be fine.
(Note: Nikon's 18mm lens has a diagonal angle of view of 100 degrees on FX/film bodies and diagonal angle of view of 76 degrees on DX bodies; the 28mm lens in FX/film has a diagonal angle of view of 74 degrees.)
If you are asking about a lens with an angle of view similar to an 18mm lens in FX (or film), then you are looking at a lens like a 10/12 -24mm zoom. Nikon, Sigma, Tokina make great 12-24mm lenses; I think the 10-24mm lenses (more recent versions)