Autofocus Problem in Nikon D5100?

When i try to focus it just stays unfocused. I've checked every thing i.e the auto/manual switch on the lens, the autofocus setting in the menu etc.

Does it look unfocused through the viewfinder, or do the pictures you take look unfocused on the lcd screen and on your computer?

If the pictures look fine on the lcd screen and computer, then you simply need to adjust the diopter dial, found next to the eyepiece of your camera.

If the pictures are unfocused in all situations, then check to see if there's a smudge or fingerprint on the front OR back element of the lens.

Finally, are you using an AF-S lens with the required autofocus motor? An AF, AF-D, and such won't autofocus with your camera.