I'm no expert, but I'd suggest looking at what you want to add and researching their requirements.
The biggest difference off the top is lighting. The organisms in a reef tank will have greater light requirements then fish only. How much light depends on what kind of corals and such you want to keep.
Another difference is water quality. You want stable water for a reef. Corals can't withstand fluctuations in PH, and salinity as well as fish can. (Not that fish like the fluctuations, but they can survive them better)
Corals will often have nutrient requirements like calcium and such and need the water supplemented with said nutrients.
If you don't have a sump, you might want to start with that. It will give you the space to add the equipment you need, and add extra water volume to help keep water quality stable.
Edit: also you want to look at the fish you have and any fish you want to add and make sure they're "reef safe". Some fish will damage and kill your expensive corals or other reef inhabitants. If you have any of these consider trading them or moving them to a different tank before adding corals.