males are usually more colourful and prettier than females and usually pet stores segregate them as they breed profusely.
guppy's give birth to live young, not lay eggs, and you should have 2-3 females for each male as he can stress the girls out when they breed.
once the females give birth, they can eat the fry (young) so a breeding net or trap (get it from most pet stores) is highly recommended unless you have lots of fine leaved plants the fry can take cover in.
be prepared to be over-run with guppies tho, as they can spawn young every 2-4 weeks and up to 50 young each time!
