You may have picked the wrong fish. Some fish are very territorial, like the Betta and most Cichlids. Male livebearers, especially Platies and Swordtails, will sometimes hate each other, and one will dominate all the rest and eventually kill them.
If any two fish you have are fighting, take one back to the fish store unless you can move them to a new tank. (ideally, take the aggressor back)
Sometimes, even supposedly peaceful fish don't get along. Adding a school of Zebra Danios (keep in groups of 6 or more) will often calm aggressive fish.
Also make sure you have at least two female livebearers for every male, or your one male will severely over sex your female. (male livebearers have a thin pointed anal fin while females have a larger frilly anal fin)