Nikon d3100 not focusing?

It takes pictures perfectly fine, until i try to take a picture of something smaller. Like when i try to focus it on an eye, for example, it just won't take the picture and it won't focus.
what do i do?

If you read your manual, you will see that each lens has a certain focusing range.

The closest you can focus with the 18-55 mm lens is around 10 inches.

For real close-up photography, you will eventually want to buy a macro lens, like the Nikkor 60 mm f/2.8

For now you can buy some simple diopters that screw on to the front of your lens (+1, +2 and +4 kits cost around $40) or an extension tube that fits between the lens and camera (about $50)