What kind of setup are you aiming for? Fish only? Fish only with live rock? Reef setup?
Let's see here....
you'll need Lighting... but the kind of lights you choose depends on the kind of setup you go for. I usually don't leave the lights on most of my setups, with the exclusion of my reef tank. I'll leave them on while watching or with guests over, but the kind of lighting you choose depends on the specific need of the fish/corals/inverts etc. you have. Obviously a reef tank will require strong lighting.
Then you need filtration. Personally I love canister filters, load 'em with media and stuck 'em below the tank. You'll also need a protein skimmer, and additional filtration if wanted or needed.
Substrates should be simple, I use crushed coral, it works great for maintaining the high pH that salties love. Live sand is great, too. Also, if you choose a course substrate, you may need to include a patch of softer substrate if you purchase fish that require it (for example rockmover wrasse and sand-sifting gobies).
Heaters are optional depending on the kind of tank... obviously a tropical tank would require a heater, and a tank for a catalina goby would not.
I would also suggest the use of a powerhead or too to keep the aquarium circulated.
You'll need equipment to monitor all of your parameters, ammonia, nitrite, nitrate, pH, calcium, etc.. ideally just buy a huge kit with everything. (NOT test strips!) Oh, and hydrometer too.
Personally, I would recommend you use a UV sterilizer as well!
