You will need a heater, and a thermometer, most salt water fish are tropical and will enjoy a temperature of 80-85, though some will go as low as 70, and as high as 90. Find out which fish can live together, even if they are similar in size, not all fish will get along. You will need to buy aquarium salt. Most pet stores will advise 1-2 teaspoons of the salt for every ten gallons. You will need a syphon and a bucket for cleaning out your tank, you will need a filter, and with it the carbon and regular filters, you will probably need airation, as most fish need air. You will also need to have food, most fish enjoy flaked food, but there are also pellets and dried worms, make sure you ask the pet store where you buy your fish which is best for the type of fish you are buying. It is also good to have a water conditioner, this helps keep the "slim" layer on your fish when they are stressed, and helps them adjust better, you will want to do this while setting up your tank, even if you don't for later tank changes. Also, have the water sitting for TWO DAYS before you buy your fish. Some pet stores won't guarantee their fish if you haven't got water that has been sitting out for two days. It would be good to have a net as well to get dead fish, or unwanted items that may fall into your tank out. You may also want a smaller tank incase you end up with sick fish, most people call this their hospital tank. I never had one, I was lucky that my fish only ever got ick and nothing else, but it would be a good thing to look into once you have your main tank set up. Good Luck! :)