Actually my suggestion to use a simple diode rectifier for such a small voltage range was incorrect as standard diodes have a forward voltage drop of around .6vdc so you will lose most of your measurement range. You are going to have to research op-amp circuits for 'precision diode' application so you can measure all the way down to 0 volts DC.
As far as the parameter to use for the analogReference command, it depends on the specific arduino board type you have. But the answer is in here: