What type of saltwater tank would you like to keep?
Fish Only = FO
Fish only with live rock = FOWLR
Fish and Inverts
Reef Tank
One method would be to use crushed coral or sand. In most cases sand is better for its expansive surface area and ability to hold bacteria. Then if using rocks, live rock would be a good choice at about 60 to 70 pounds in a 55 or whatever you desire but more is better unless you choke the tank with it. Then you could use something for machanical filtration....ie... Saddle filters, canister filters, wet/dry and or sump. A wet/dry is prefered, this will allow the tank water to leave the tank and be proccessed and filtered, and possably by a protein skimmer in sump and then back to the main tank. Or you could have saddle filters on the back of the tank like Penguin 400's and a hang on protien skimmer. Then you will need lights (which I would think about first as it will cost a fair penny depending on what type of system you would like to have).
There are alot of ways to go here every setup is different.
You will need to check out the DIY section on how to build your own equipment.
Thomas