what kind of lens

i have a nikon d3100 and was wondering what the best lens would be to get extream close ups like eye shots

A macro lens or magnifying filters. The filters will be cheaper you'll just need to find ones that fit the diameter of your lens.

so they have magnafying filters that just snap on any lens as long as it is the correct diamiter

If you need quality filters are not the way the 3100 I believe needs a af-s or af-i to work fully that means the highest quality and fully compliant would be the nikkor 105 2.8 VR macro