Why won't my nikon 50mm 1.8 autofocus?

I just got a new lens (nikon 50mm 1.8) and yes it is labeled as AF. But no matter what I do, my camera stays on MF. I have a nikon d3100. The tab that you can switch is down, where the two orange spots meet. This is the AF setting right? What am i doing wrong?

The lens is an AF one, but that's not enough - this just means that it supports autofocus in general. The 50mm/1.8D lens however depends on the old autofocus motors that use a screwdriver like connection. Your camera has no such motor, it depends on the lenses to bring their own internal autofocus motor.

Nikkor lenses that have such an autofocus motor are labelled as AF-S. Third party manufacturers use different designations.

There's no auto focus motor built into your camera or lens. You need AF-S lenses with your camera. Lenses with motors in them since your body lacks one. The 50mm lens that will work with your body is the 50mm f/1.4 AF-S. It's about $400-$500

The first two answers are correct.

But don't get stressed over it. That is a terrific lens! And if you're new to photography it's a great one to learn with. It's also very good for shooting video.

Enjoy it!

The three answers above me are correct, but check out "manual focus" in your owner's manual. Read the part about "electronic rangefinder." If you are afraid to go it alone with manual focus, your camera will still confirm that you are in focus by giving you the beep and the "in focus" indicator in the viewfinder.