It kind of depends on what you mean by "breed". If you mean get them to have babies, that's pretty easy. But if you mean you want to actually breed them and create a stable strain etc. That is a different matter and takes a great deal of work. To breed for a strain you have to choose carefully which fish to keep and breed to pass along desirable traits to their offspring.
If you just want awesome little guppy babies I generally find having at least 7 strong adult females and one adult male does the trick. I've had as low as 5 females but found that with that few they tire quickly.
You will also need a place for the young to hide or a breeding area/net for them to be separated in. What you do in this regard will depend on your desires. I just wanted them to have babies and for some to survive. so I had an area in the tank that the little fish could go and the big ones could not. Those little ones smart enough to find it and stay in it lived and those that didn't got eaten.
Having fry of any kind in a tank is, for me, a great joy.