Zim's appearance, like that of most of the Irken race, seems to be based off the typical "little green men from outer space" stereotype. Many Irkens are about the size of a preteen schoolchild, but Zim is exceptionally much shorter. Zim also has ruby-colored eyes; a very common trait in Irkens. It was also mentioned in the episode Walk of Doom that Zim's eyes contain ocular implants. Since the eyes he was born with are not visibly different from subsequent appearances, it can be assumed that these implants are subcutaneous. As he considers himself to still be an Invader, he continues to wear the standard uniform: a red tunic-like garment with a pink collar and sleeves, as well as black boots and black gloves.
Note that most of this information is from The Invader Zim Wiki.