So what equipment do you NEED?
The problem here has to do with the size of the tank, what your goals are and how much space you can devote to the system, how much time you have to mess with it, how much money you have and how much you are willing to get into the nitty gritty of testing, chemistry, electronics, plumbing and so on.
That being said, here is a list
Aquarium preferably drilled
Sump - really nice because you can put equipment there where it is not causing an unsightly mess in the aquarium
Powerheads - create flow in the aquarium. It is nice to have high end ones that can vary their flow
Skimmer - removes wastes from the tank
Filter socks - not necessary but they do remove fine particulate matter from the water column increasing clarity in the water
Heater or two - keeps the aquarium temp in a narrow band. Can get stuck on the "on" position and cook the tank
Rock - provides a place for bacteria that break down ammonia to nitrites to nitrates
Plumbing and return pump if you have a sump
Misc - nets, brushes, algae scrapers
Quarantine Tank - you hold new fish in this to get them used to eating the food you feed and to check them for disease
RODI system with storage tank - produces highly pure water from tap water
Container to hold mixed up salt water
Auto Top Off System with storage tank
Lights to illuminate the tank for a Fish Only system and Really Strong Lights if you want to have hard corals.
Test Kits and Refractometer - for determining if your water conditions are OK