For breeding guppies, you need 3 tanks, one 5 gallon and 2 10 gallon (if you want to save the babies that is).  First, you get 2 female guppies and 1 male.  Put them in a 5 gallon tank with all the supplies for a tank. (Filter, heater, gravel, light, net, etc.) Then after the females are pregnant, take the females out of the 5 gallon tank and put them into a ten gallon tank. When the females have the fry, take out the adult females and put them into another 10 gallon tank.  After the babies grow older, and you can tell if they are female and male, put the males into one 10 gallon tank and put the females into another.  With the adults, you can move the females back into the 5 gallon tank and get a divider so that the adult guppies won't breed again!
Hope that helps!!  
Guppy Luver