Hello Angel. Just a few "always" things to know. Always:
Get a large tank as "kom" said and make sure it is much wider than it is tall. Fish swim horizontally, not vertically. Those tanks that are 4' tall and 1' wide are torture to fish of any kind.
Make sure that the water is perfectly adjusted before ever putting in the fish. Wrong salinity or "PH" will hurt the fish.
Make sure the heater and the pump are powerful enough to do the job they're designed to do.
Make sure that the fish are compatible! Fish eat fish and some are VERY territorial. They will torment or even kill other fish that they feel are a threat to them.
Make sure that there aren't too many fish in the tank and allow for their growth.
Make sure YOU are ready for the maintenance work. A salt water aquarium is a BIG undertaking.
Keep kids away from the tank! They love to tap on the glass and throw things in it.
There are plenty of books and online helps available.
Hope this helps.