You'd essentially have to empty your 55g of all its content, give it a good clean. Some people simply use hot water. If you have mineral build up on the tank, plain white vinegar will take it off pretty easily. Some people use hydrogen peroxide to disinfect and some even use pure lye (sodium hydroxide), but it's a lot more dangerous to handle. Never use detergent/soap, however.
After that, you can start setting up the reef. A 55 gallon can keep a wide varienty of reef-safe fish. Honestly, there is too many to list, so it's a better idea for you to ask us about any fish you're interested in and we can tell you if it's okay for your tank.
What types of corals you can keep is largely dependant on your equipment, especially lighting. The three pieces of equipment I'd never skimp on is good lighting, a good protein skimmer, and a RO/DI filter for your freshwater (used to top-off the tank or mix saltwater with). For a 55 gallon, I'd go with either 2 x 150-250 watt metal halides with actinic fluroescent supplementation, 4 x 65-96 watt PowerCompact fluorescents, or T-5 lighting. For the sake of simplicity, I'd use a hang-on back protein skimmer such as the AquaC Remora Pro. RO/DI (stands for Reverse Osmosis/Deionization) units are usually about the same in quality, so as long as you have a RO with TFC membrane, you're in good shape.
On top of these, youll need adequete cirulation and a heater. A couple of powerheads will do just fine, and you can put them on a wavemaker if you so choose (they help alternate the flow).
If you'd like to know the details about setting up the tank, just ask
There's a good amount of info online and in books that details how to set up a reef tank.
Here's a good set of books we recommend for beginners. Most of them have very good, insightful tutorials on how to set up a reef tank step by step.
http://www.reefs.org/library/reading/be ... inner.html
This is a good read if you haven't seen it yet:
http://www.reefs.org/library/newbieguide.htm
Let us know if you've got any questions
Welcome to this terribly addicting and fascinating hobby!