Put your platy in a separate tank or a small plastic container in which she will give birth to...20-30 baby platys. She should be alone, because she might get disturbed by other fish. Once she gives birth to all the babies( you will find out once her belly is thin and about 3-4 hours have passed), remove her from the container into another tank. While she is giving birth, put some plants( moss is the best) in so the babies can hide from being eaten by their mother. For a day, you don't have to feed your babies because they are still feeding on their yolk sac. After a day you can start to feed them crushed to powder flakes. I use Hikary Fry flakes... I don't remember the name right now, anyway, they are very high in vitamins and I suggest you use them. After 1 month the babies would be 1-2 cm. in size and you can start feeding them small pieces of bloodworms or brine shrimp. After 1 more month passes, they are full grown adult platties!!!