Did you set the tank up before you got the fish?
Did you add a dechlorinator to remove chlorine/ chloramine?
How long were the guppies in the bag for when travelling home form the pet shop?
How did you acclimatise the fish to their tank?
Increase aeration and check ammonia, nitrite, nitrate and pH levels in the tank water. Do a 75% water change and gravel clean the substrate if there is any ammonia or nitrite, or if the nitrate is above 20ppm.