Tascam Guitar Trainer

ok it said i had to put a zinc nail and copper wire in a fruit and i could use a voltammeter to find the electrical current the fruit would make. BUT a guy at radioshack told me that this thing called a multimeter is a voltammeter. Ok it has to things attached to it so i put them in the fruit thinking they could replace the copper wire and zinc nail. I got results but some had a - sign next to them. And there's like a whole bunch of settings. There is DCV, AVC, OHM, and DCA and sub setting but i understand them. BUT which do i set it to measure voltage. Plus i put them in a lime and that - sign came again so i switched them and it wasn't in the negatives anymore. SO it confused me so i was wondering which way is right and does a fruit have a negative and positive side like a battery. because of of those wires in negative and one is positive. SO PLZ HELP IF U CAN! i am only 12 i don't get this stuff! and my project is due soon.