alright what r u keeping them ina trap or a tank? to start with feed them on the commercially ground up flake food for atleat a week then i make my own concocsion of food, buy blood worms and livebeaering food in freeze dried form, then i break it up very fine, so they have a healthy diet of blood worms and tubifix worms and flake food, also do a water cchnage of there tank every three days that way they will grow faster, make sure the type of filter you have is no to powerful, i suggest you use one of those pump drivin ones and have it ona low setting, that is generally what i use. when they get to about 2 weeks old you should be able to sex them by useing a torch and putting a black piece of paper on the back of the tank, shine the torch through the whole and you will be able to see a small gravid spot on the females, i would then remove the males from the females, put them, each sex into different average sized tanks, 60litres tanks will do, let them grow and slowly ween them off crushed food into normal food, when they have grown chose what male and femal you like the most and breed them, yo start getting the best line breed of guppies you have. then do all of the above over again:D
Cheers Shae