I tried back button focussing with my nikon d3100 by going to the button section of the setup menu and pressing ok for the AF-ON button. I now want to return to autofoccusing with the shutter but don't seem to be able to. How do I do this?
The D3x00 range does not have a dedicated AF-ON button, so I presume from your description that you've enabled back-button focusing by assigning the AE-L/AF-L button to the "AF-ON" setting. When the AE-L/AF-L button is set to "AF-ON", the shutter-release button cannot be used to focus.
To reverse this, you will need to go back into the setup menu and use the "AE-L/AF-L Button" menu item to select a different action for the AE-L/AF-L button, which will re-enable the autofocus functionality on the shutter-release button. I believe the default setting for this button is "AE/AF Lock", but selecting any of the other AE lock or AF lock modes (i.e. anything other than "AF-ON") should also allow you to focus using the shutter-release button again.
This information can also be found on page 146 of the D3100 User's Manual.