How to maintain a saltwater aquarium

1.Buy supplies fit for salt water. Get an actual fish tank, with a filter, air pump, gravel, decorations, something to do water changes with and special aquarium salt.
2.Set up your tank. Install the filter and air pump, put the gravel, water, and decorations in, and put the salt in and dechlorinating the water. Do filter cleanings and water changes weekly. Make sure the tank is big enough for the fish before getting either, and monitor the amount of salt in the aquarium.
3.Go through a "fishless cycle": a week where you "take care" of your aquarium with no fish in it. This will get the tank ready for the fish and give you good practice. If you're a bit extra paranoid you could also set up a pretend aquarium for more practice before you set up the real fishtank. (A good way to do this is to fill a plastic drawer with water, put two toy fish in it, use ripped up napkin in an empty water bottle for fish food, use a dirty cup for the filter, use a box for the air pump and water with soap in it for the dechlorinating materials...)
4.Decide on which type of fish or multiple fish you want in your aquarium, then research those species. Make sure they are saltwater fish, not brackish-water fish! For example, while clownfish and yellow tangs are saltwater fish, but Black Mollies and Dalmatian Lyretail Mollies are brackish-water fish with very different water needs!
5.Look for a place that gives away fish. A pet store could be a place to start but isn't always the best place to get any animal. Rescue groups are rare for fish but they do exist.  If you do decide to get a fish from the pet store, make sure it and the other fish in the tank are healthy. If any fish have white spots, irregular swimming patterns, or anything of the like, it is a sign of illness. Don't get the fish there if the tank water is dirty or something either, as it can cause major sickness in fish.
6.Get your fish. Bring them home in the bag- careful not to pop it!- and then hold the bag in the tank water for a while so the fish gets used to the new water temperature.  Put some tank water in the bag. Then slowly let the fish swim into the tank. Your fish is home!
7.Take very, very attentive, careful and good care of your saltwater fish- this will increase its chances of a longer lifespan. Work on being extremely observant, since illness can be very hard to spot in many animals. You might even want to install a second filter for extra cleanliness of the tank, though this will mean more filter cleaning every week.
8.Know that, if you take really great care of your fish and their aquarium home, you should be able to maintain a colorful saltwater aquarium. However, all fish die. If your fish does die, by all means do not flush it down the toilet. Drains, unlike in the movie Finding Nemo, do not lead to the ocean: they lead to the sewage system. It isn't as much of a "burial at sea" as it is a "burial in the sewers".