Firstly, will you be keeping soft or hard corals? If you don't know the difference then you need to read a good Reef keeping book and a few threads on a good forum like Ultimate Reef. The reason i ask is because your lighting, feeding, and knowledge of water chemistry is going to vary, so is the cost of buying, the availability and care of the corals.
How much are you prepared to spend initially on setup? And more importantly, how much are you prepared to put aside every month financially and time for maintenance. Keeping a reef aquarium is very expensive and a time consuming hobby. Running costs can run really high, think about it. Running a four foot tank with two 250watt metal halide lights on for between eight and ten hours a day, with two or three pumps each using 50 to 60 watts on 24 hours a day is going to use a fare bit of electricity.
Then there is water changes, You will need to do a minimum of 25% a month, many do 10% weekly. You may have to get this from your local fish shop. Many reefers produce there own with the use of a Reverse Osmosis unit. But still a salt mix requires to be purchased.
Feeding is the least expensive part of this hobby, but still it it nice to get a good variety, especially for the living corals. If you decide to keep hard corals that is. you need to be able to feed your fish, invertebrates and corals.
Water chemistry can become very expensive. It is very important and if you cannot afford these in part of the initial outlay i would not bother taking up this hobby until you can. You will need test kits and buffers to adjust and maintain the correct levels if keeping a heavily stocked hard coral reef. Stocking slowly is very important, but i would still get all the relevant test kits, that way you can at least test for it. Then do something about if the need arises.
Test kits include Salinity, Nitrate, Phosphate, magnesium, Alkalinity and Calcium as a minimum. Then you may need the buffers to adjust these. I would before starting visit a marine supplier and get a idea of the costs of these. You also need to gain a understanding in these and take precautions when dosing because dosing too fast can cause many severe problems.
This article all sounds really off putting, but these things really need addressing so expensive mistakes are not made. But the good news is with plenty of reading and research there is no reason why a successful reef tank can be created in the comfort of your living room. New advancements in technology like LED lighting really help with the costs of reef keeping.