It really depends on what you want to keep? Do you want just fish or fish and corals+inverts?
Have a look at the links below to get you started and give you a better idea of what you want to do:
<a href="http/www.fishforums.net/index.php?showtopic=138236" target="_blank">http/www.fishforums.net/index.php?showtopic=138236</a>
<a href="http/www.fishforums.net/index.php?showtopic=138425" target="_blank">http/www.fishforums.net/index.php?showtopic=138425</a> (handy list of abbreviations that are commonly used
).
I would say have a look at a few online marine shops and through the tank journals on here to get an idea of what you like and what you would want to keep. Then let us know where your wanting to head and we can be a bit more specific about what you need.
Basically for Fish only with LR (FOWLR) you need:
Tank
Skimmer (Optional, with big messy fish though I would say its well worth it)
UV (optional but with a 200g tank thats going to be a lot of money to stock so worth doing everything to protect it)
Liverock - Main method of filtration, for a 200g tank you will need a lot of this.
Water flow - You will need a decent amount of water flow (at least 10 times your tank volume to make the LR filtration work).
Heater - You will need a couple of decent heaters for a tank this size
Lights - For a FOWLR lighting doesnt really matter to much
For Fish and corals (reef) you will need (very basic list):
Tank
Skimmer (optional but I would say needed even more for a tank with corals)
UV (again optional but like above thats a lot of fish stock in a 200g so worth protecting)
Liverock - same as above
Water flow - Here is the first difference. For most soft corals you will need at least 20 times your tank volume in water flow per hour, more hard coral (LPS and SPS) you may need up to 30 times flow.
Heaters - As above
Lighting - Lighting is very important for a reef set up. What kind of lighting you get depends on the dimensions of the tank and what corals you want to keep. You are probably looking at around 3 250w metal halide units for a tank this size.
Other equipment common to both setups:
A good brand of salt (buy a large bucket of it)
Hydrometer or refractometer (for measuring salinity), personally I find refractometers easier to use and more reliable.
RO unit (definitely needed imo for a reef tank, not so much for FOWLR).
Test kits (ammonia, nitrate, phosphate and nitrite)
There are a couple of methods you can use for water flow depending on what you want to keep, how the tank is setup and your budget. You could just use a few powerheads in there which will work. On a tank this size though personally I would invest in 1 or 2 decent wavemakers as they will not cost much more then the powerheads you would need and give much better water movement.
You can also drill the tank for a "closed loop" system. Basically this has a large pump outside of the tank that pulls water form the tank and pumps it back via pipes placed around the inside of the tank. Takes up less space then powerheads but is more difficult to set up (again I would go the wavebox route).
The other thing you can add is a sump. Basically this is a tank that sits under the display and can be hidden away. In here you can hide equipment such as skimmers and heaters. It also adds extra water volume.
The last thing I would strongly recommend for a tank this size is a quarantine tank for your new purchases. Eventually you are going to end up with a LOT of money invested in livestock and the last thing you want is to add a fish that has something wrong with it that ends up killing everything in there. a QT will allow you to make sure the new livestock you buy is 100% healthy before you add it to your tank. It also gives you a place where you can separate any sick fish for treatment (as most medications can do serious damage to a marine tank).
Hope that helps get you started. Like I said the best bet is to have a look at what everyone else is done and get an idea of what you want. Once you sort of know where you want to go feel free to ask as many questions as you need.