well first, I would like to know what type of guppy, but no matter which type, a guppy is a livebearer, meaning, the fish does not give birth to eggs but the fry/fish are on their own as soon as they are born. In other words, guppy fry are naturally born swimming.
The adults most likely will eat the fry especially the parents of the fry. So if you really want to keep the fry, then you must place the pregnant guppy into a tank of its own. You buy a whole 5 or 10 gallon tank, but it must be cycled or you can buy a breeders cube or tank, about 1/4 of a gallon large which stays in the water at the surface of the main tank, so it is warmed by the main tank water, and when the mother gives birth, the fry drops through small holes so the mother is unable to eat them. Then you take the mother out, and you feed the fish until they are about 1/2" or so or can be placed back into the main tank not to be eaten by other fish. Baby brine shrimp is the best all around fry food, you can also give them pulverized fish flake which especially small enough for fry. But baby brine shrimp (which are eggs kept in a jar in the refrigerator) is the best way to go.
I have red platys which are livebearers. All the fish I have in the tank were born into the tank. Now that I do not want anymore platys, I keep african dwarf frogs in the tank for these frogs will snap up any and all platy fry which the mother or other adult fish do not.
Let me/us know how it goes.