A 5.5 will be heavy on evaporation, and more difficult in terms of temp, salinity, and param swings. For a first SW tank, it is often a good idea to start with a larger tank such as a 10 or larger, however if you are set on a 5.5 it is definitely doable. In fact, my first tank is a 3g reef
and I can tell you it is a lot of work.
Also, what kind of tank are you thinking about? Full-on reef, with coral? In that case, I recommend starting off with some easier soft corals such as mushrooms, xenia, leathers, and zoas.
You'll need 3-5 lbs rock (live or dry, I prefer live, especially for pico tanks), sand (live or not, I used live), a powerhead, reef capable lighting, a heater, thermometer, a refractometer (or hydrometer, which can be more inaccurate), and reef grade salt. I'm probably forgetting something, but I'm sure others will chime in.