In that case.....
There are three important things to consider ahead of time.
1) How big the tank will be.
The bigger the tank the easier it is to maintain proper water quality, but big tanks are also more expensive to filter and light.
a) Reef Ready (drilled). A drilled tank has a built in over flow box that allows you to put the filtration under the tank in the stand, usually used with trickle filters, or sumps, it also increases the over all volume of the system. Much more convenient, but also more expensive.
b) Standard. A standard tank is not drilled and you are limited to hang on the back type filtration. But they are much cheaper.
2) What kind of lighting.
Two main types for "reef" tanks. Power compacts and metal halide. Power compacts are inexpensive, don't put out very much heat, and work out pretty good for smaller tanks, but they somewhat limit you on what you can keep, because they aren't quite intense enough for some of the more light demanding corals. Metal halide are very bright, very realistic looking, and do well on larger tanks, they are more expensive (in most cases), and put put more heat, but you can pretty much keep anything under them.
3) Filtration. Depending on the size of the tank and what you put in it, there are a few options.
a) Reef ready (drilled). A sump allows you to use higher capacity filters or protein skimmers. If you don't use live rock, a trickle filter will work, but will limit you to fewer fish in the system. If you use live rock, a simple sump (empty box of water) will work fine, with a high powered protein skimmer.
b) Standard. If the tank is not drilled, you are limited to hang on back equipment. Hang on protein skimmers are not as powerful as in sump models, but are still necessary. If you are not using live rock, a hang on filter or canister filter will work, but you will be limited in how many and what types of fish you can keep. If you are using live rock, then a good hang on protein skimmer would be all that you would need.
Hope this helps!