What are and how do use exposure meters? - 1

I have a Nikon D3100 and I don't know how to use exposure meters. Please help explain what they are and how to use them?

Start by learning how to use the one in your camera. Get the manual and read

Your camera has one inside and this is the first thing to learn about when handling it. It measures the amount of light the film, or in your case sensor, will receive. You can use aditional lightmeters, but first use properly your camera