as it was said before, we will need more info to help.
what size tank do you have/plan to get?
do you plant to get a heater?
do you plant to get a filter?
do you want to breed?
do you understand what "cycling" a tank is?
if you want to breed, do you know what you would do with the babies once they grew? even if you got just one female that you didn't think was pregnant, if she was kept with males, she is. guppies also store sperm for up to 6 months, one batch each month. say she is a small fish and has 10 on the first drop. each drop they have 50% more than the last drop. so #1 would be 10 babies, 2 would be 15, 3 would be 22, 4 would be 33, 5 would be about 50, 5 would be about 75!!! the number varies for each female, and there is only a certain number that each female can hold at a time, but if that chart were exactly correct for the individual you bought, the total would be...over 200 fry!! lets say that, for each batch, 2 fry die. we are still talking around 200. it is common to sell the fish as adults to the fish or pet store, but that may not be an option for everybody. so one has to really think if they want to breed or not.
do you have any tanks/fish already??
would you have an all guppy tank, or would you mix other fish in?
what type of fish would you put in, then?
are you interested in showing guppies?
please be sure to answer all of the questions.