You will need:
Copper - coin
Better than regular wire
Zinc - clove
Copper - coin
Better than regular wire
Zinc - clove
From 1 lemon - 1 Volt (In a regular AA battery 1.5 Volts)
From 2 lemons - 1.8 Volts (A little more than a regular battery)
From 4 lemons - 3.5 Volts (You can connect a light bulb)
Good luck