There are two ways to go about it. Pick the corals first and build to suit their needs, or build a basic tank and find out what corals you can put in there.
I did my first tank based on a couple corals that I really wanted. Then set it up and spent about a 10 months maturing the tank to support those corals. Until then I added other corals that would also fit my setup.
Lighting wattage,temperature, and type depend on the types of corals you want to keep. It also depends on how deep the coral will be in relation to the light.
Go to a SW store or online store that sells corals and look through it to see what you like. Some corals will not do well in a smaller tank. Once you pick some corals that you like you can ask here or search other places on the web to find out the lighting and general care requirements for that type of coral.
Also check to see whether a coral is aggressive and will harm other nearby corals.