How to find the autofocus on the nikon 50mm 1.8 lens?

This is the exact one i had ordered

http://www.amazon.com/...00005LEN4/

i read some of the reviews and people talk about how great the autofocus is.
how do i find it?
all i know is my lens is like a manual and i don't know how to work the autofocus.
is it a setting on the camera?
i have the nikon d5100

Picture Take is correct, except there's no such thing as a 50 1.8 AF-S DX. There's a 50 1.8 AF-S and 50 1.4 AF-S.

That lens will NOT auto-focus with your D5100. You need an AF-S lens.

You could get the 50mm f/1.8 AF-S, 50mm f/1.8 DX AF-S, or 50mm f/1.4 AF-S.

See the DX lens here: http://www.amazon.com/...001S2PPT0/

Although I'm not seeing the DX lens on Nikon's site. Perhaps it was discontinued. I remember thinking it was kind of silly, because it was not much cheaper than the FX lens.