For this you will need a zinc plate and a copper plate . Insert both plates in potato .connect one wire with zinc plate it will act as negative terminal and one wire to copper plate which will act as positive terminal .Now connect both these wires to a ammeter with help of which you can read the current flowing . The amount of electricity produced is too less so you can't glow a bulb but still if you use number of potaoes [may be 4] a bulb can glow . One potato can be used for something which requires less voltage may be a digiclock .Instead of potato you can also use tomato or lemon .