Sorry for my English, I'm Brazilian so I do not speak English well.
Step 1: Hunt for materials
One or more penny
1 nail or screw (1 nails or screws (or whatever is made of a metal other than the money)
A multimeter or a voltmeter
Step 2: The fruit
Make two cuts in the fruit, so we can fit the electrodes.
Using a lemon Galician and this structure got a voltage of 0.83 volts, but you can connect more batteries one other, thus increasing voltages.
Important: If you connect two or more batteries, make sure that the contact between them is perfect, but you will get with many very small voltage batteries together.