How to turn on autofocus on my Nikon d3100?

My camera won't allow me to switch from manual focus to auto focus. It says "this option is not available with current settings". My camera is set on manual mode, auto white balance, iso 100, matrix metering, and with active D lighting on. What settings do I need to change in order to be able to switch to auto focus? I've also tried resetting my shooting settings but nothing changed…

Set the lens focus switch to Auto as illustrated on page 62 of your manual. Then you can set the Focus Mode as explained on page 55 of your manual.

All of your answers are in the manual.

With the AF-S lenses (which your 18-55mm kit lens would be), selecting between auto or manual focus is done with the switch on your lens. If the lens you are using is not AF-S and lacks that switch, it will be manual focus on your camera.
I'm guessing that you are referring to your camera menu. I have a D90, so the exact selection will be different, but some things should be similar.
If I go to Custom Setting Menu, Autofocus, top of the list is AF-area mode. That gives me choices of Single Point, Dynamic area, or Auto area.
If my AF-S lens is switched to auto, these choices are available. If it is switched to manual, AF-area mode is grayed out, and if I try to use my multiselector to get to the choices, it will give me "this option is not available with current settings".

Left side of the lens has a A / M setting. It means automatic / manual…
Top right of camera has a little green icon that says " auto ".
Have you read the first few pages of the owners manual?

Is the lens itself set to autofocus?

When all else fails… READ & STUDY the Owner's Manual for your camera.

Watch:
http://www.nikondigitutor.com/eng/d3100/index.html