Your female will probably give birth in several days to a week. If you look closely and can already make out the eyes of the fry in her belly then it will be very soon. Be aware that guppies do not care for their young, they eat them. The little tank you bought is called a breeding trap; you put the female inside it and when she gives birth the fry will fall through the slots in the bottom to safety. Immediately afterwards you should move the female to a seperate container and allow her to rest, don't give her too much room to move around or she will waste her energy. The fry should always be reared in a seperate tank, for two reasons: they will be eaten by the adult guppies, and adults release a hormone into the water that stunts the growth of fry. In the wild, this hormone would be diluted by the flow of water, but in a confined area like a tank they will only grow to about half size. This happened to me with my first batch. (only one miniature survived hehe) That was several years ago, I know better now.