Well Chis, that is a good question and one that many many people have many many differing opinions on. I guess you can't argue with success, and so far (8 months) my tank has been successful. So I can at least let you know what has worked for me. A few things are certain.
1. You want at least a rough idea of what you want out of the tank before you start it. Will it be a coral tank? Fish tank? Specialty tank? What life would you like to see when your tank is set up. This will determine the size of your tank, as well as your filtration choices. For example, I really wanted a Yellow Headed Jawfish even before my tank was ready for sand. This fish digs his home in the sand and needs a deep sand bed. I purposely made a deep sand bed in my tan kmonths before ever purchasing the fish. (I have included a picture of him below)
2. Some essential items: Protein Skimmer, salt (I use Instant Ocean salt), water purifier (to get rid of chlorine, etc.), filter (most people remomend a sump filter which is expensive. So far my FLUVAL filter has done the trick, but looking back on it, I would go with the sump for so many reasons.), Thermometer (one that isn't metal and won't carode in salt), Hydrometer, Heater, Power Head... I mean, most of this is common knowledge with any kind of tank, not just salt.
I used aragonite for my sand base. I also have lots of live rock in my tank (rock taken directly from the ocean with live things in and on it) but I have seen tanks successful without live rock and normal freshwater gravel on the bottom. My best advise would be to NUMEROUS fish stores in your area with salts in them. Find the one where you trust the people, the fish look healthy, and you like what you see. Ask the owner for his/her advise in starting a tank just like theirs in that store. Then, find someone, perhaps in that store, who has a tank that is set up in similar fashion who has been doing it with success for a year or so. (Make sure it's not someone working on commission and just looking to sell you the most expensive stuff. Saltwater is expensive enough!!
) Ask that person as many questions as they will allow. Go from there.
Again, numerous methods that many different people sware by as the only way... but you can't argue with success. Any specific questions you can also ask in the reef talk forum here at fishforum.net Hope this helps!
-Joe