Macro shots

Typically the Nikon D3100 is sold with an 18-55mm lens, which has 0.3x magnification,  which isn't great for macro.

A good macro lens will have 1x magnification (also called 1:1), check out LensHero, you can browse macro lenses for the Nikon D3100 there!

Macro filters can also be screwed on the end of a lens, which would be a cost effective way of getting better macro shots, see 52mm macro filters which would probably fit the 18-55mm lens.

1. use macro filters

or

2 . use extension tubes and to get good macro capability