Guppies are live-bearing fish -- they give birth to fully formed babies rather than lay eggs. Guppies are among the easiest fish to breed in captivity. Many pet stores house male and female guppies together in the same tank, so you have pretty good odds of bringing home a pregnant guppy from the pet store.
A pregnant guppy will develop a pronounced bulge in her abdomen. If you have other guppies in your tank, the difference between the sleek, slender non-pregnant guppies and the pregnant ones will be very noticeable. Your mother guppy may have a red or dark triangular spot at the base of the abdominal bulge. As the fry or babies develop, you may also be able to observe small black dots through the mother’s skin -- these are baby fish eyes.
As pregnancy progresses, a mother guppy is likely to become secretive, hiding in foliage or in commercially produced structures you have in your tank as decoration. The mother is looking for a safe place to birth her babies where they can hide from prey -- in this case, larger fish in your aquarium. If you don't have full, leafy plastic or live plants in your aquarium, add some so the guppy fry, when they're born, will have a safe place to hide.
The bigger and closer to birth the mother guppy gets, the more lethargic she’ll become. These are all normal symptoms, signs that the mother is getting ready to give birth. She may swim slowly and not eat as much as usual, though it’s important to offer adequate food so she can still eat, even if she’s no longer the fastest swimmer in the tank.
If you want to make sure your guppy fry survive, consider putting the mother in a breeding tank. This is a small plastic box that attaches to the inside of your aquarium and contains an inverted split at the bottom for baby fry to safely drop through after birth. This allows the fry to grow without the potential of being eaten by their mother or other tank mates. Return the mother to the main tank after birth, and feed the baby fry flake food immediately. Keep them in the small tank until they’re large enough to survive in the community aquarium.