Start with only a few fish for the first month.
Choosing your fish: Your aquarium can support 2 inches of fish for every gallon of water, i.e. a 20 gallon tank can support 40 inches of fish, the number being dependent upon the size of the fish. In general most fish will get along with each other if you simply remember that big fish make their living eating little fish.
Introducing fish to the tank: When you bring your fish home, float the bags for 10-15 minutes to equalize the temperature between the bag and the tank. Then open the bag the fish are in and mix water between the bag and the tank several times over a 10 minute period. In this way, your fish will be gradually acclimated to the water of your tank.
Always feed the fish in the tank prior to introducing a new fish and then turn the light off for one hour.
A ratio of two females to each male is preferable with live bearers like guppies, swordtails, etc.
Have three or more of each schooling fish, e.g. neon tetras.