It's perhaps also worth saying that while breeding cheap-and-cheerful guppies is easy enough, breeding good-quality guppies is not easy.
Specifically, you want good quality stock that are "pure bred", so that they're offspring look like the parents. Often, only males are sold for some varieties, because the exporters don't want you breeding that kind of fish. So you need to hunt down a couple of males and half a dozen females of the one breed (red cobras, snakeskins, or whatever).
Since females store sperm, you want virgin females that have never been with males. There's no point paying good money for a female red delta only to find out it produces broods with a mish-mash of characteristics from who knows how many males.
Then, you need to choose good quality fish. Look for things like deformed spines, misshaped fins, blotchy colours, and so on. Read up on the variety first, so you know the "show quality" features.
Females produce only as good quality offspring as they received good quality care. This is conditioning. It's a good idea to isolate the females from the males and fatten them up beforehand. Frozen food is good, live food is best, but quality flake will do. Make sure you don't overfed them, though.
Once you mix the males and females, nature will take its course. Your job will be to remove babies to the breeding tank. Since guppies will eat their offspring (they'll eat any small animals in fact), you won't be able to raise the full broods without a breeding tank. People who leave the babies in the tank invariably wind up with only a few babies. Guppies can produce anything up to 70 babies per brood (though 20-30 is more normal), so a 5-20 gallon breeding tank is ideal.
Livebearer babies often need to be sorted, with deformed ones destroyed humanely (feeding them to a big fish like an angelfish is one popular approach!). Regular water changes, frequent meals, fairly warm water, and good filtration are all essential to getting baby livebearers to grow to their full size.
Bottom line: lots of people think livebearer breeding is easy. On one level it is, because the babies are big and easy to feed; but on the other hand doing it right is a real art-form. But very rewarding. Good quality, locally-bred livebearers are easy to sell back to retailers or swap with other livebearer enthusiasts. Raising a big brood of livebearers is wonderful to watch, and once you have the simpler species done, there are so many rarer species you can try out as well, from pike livebearers to halfbeaks. You can even try and create your own varieties of guppy or whatever, a real challenge. Livebearers are NOT boring, and they're NOT just for beginners!
Cheers,
Neale