Your female guppy will already be gravid (pregnant) due to mating with the male in the tank and possibly other males she was with before you got her. Male guppies impregnate the females with sperm packets. The female can carry 6 or more sperm packets from various matings with different males and use one packet per batch of eggs. The females can carry these sperm packets around for months and use them to fertilise a batch of eggs when conditions are good. The eggs develop and hatch inside the female and the fry spend several more weeks inside the female developing further. After about 4 weeks, the female gives birth to a batch of young and it can vary between 10-100 fry, sometimes more, sometimes less. Young females have smaller batches about once a month. Older females have bigger batches about every 6-8 weeks.
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If you want to breed guppies, put the female in a well planted tank without any males. Feed her well and when she is ready, she will give birth. The babies will hide among the plants and if the female is well fed, she won't normally eat the babies. Most females don't eat their young unless they are stressed or really hungry.
The best plant for baby livebearers (guppies, platies, swordtails, mollies) is Water Sprite (Ceratopteris thalictroides/ cornuta). It is a floating plant that has lots of branches and nice fine roots for the fry to hide in. You can also plant the Water Sprite in the gravel if you get too much and it grows into a lovely light green shrub.