the most essential thing you need is patience.
You cannot add sand & rock to a tank and toss fish in there a few days later
sw tanks need to go through the nitrogen cycle before adding any livestock to them. Are you familiar with this process?
(before i spend time explaining it)
the most important thing a person needs to do before starting a sw tank - is a lot of reading. At the top of the reefs for beginners forum (you can find it on the blue menu bar on the home page) are a bunch of 'stickies' that contain very important info. If you haven't read them, you are starting off with far less info than you should have at this point!
some facts for you:
-->you can't mix sw properly if you don't have a heater..the salt mix won't dissolve properly
--> if you start your tank off using tap water, you will have major algae issues down the road as your new rocks & sand will adsorb all of the garbage that is in our local tap water.
You asked what you need to get started - at an absolute minimum you need:
--test kits for ammonia, nitrite, nitrate and ph
--a refractometer to properly measure your salinity
--a ro or ro/di unit to make your water - or a tap water filter (not really recommended but acceptable if you can find an ro yet)
--a heater that is the correct size for your tank
--a thermometer
--either a skimmer or a filter that will help oxygenate the water (assuming you have no sump attached to this tank?) a canister filter will not add enough oxygen to the tank and are only useful on a reef tank as a vessel to add media to (like carbon or gfo). You'd be better off with a hob filter for the meantime before you get a skimmer
--a good book on this hobby (the best $ you will ever spend in this hobby!!) i suggest
the conscientious marine aquarist by robert fenner
...i'm certain i am leaving something off of this list...