This question really requires us to know details about your setup. Do you have live rock and live sand? What type of filtration? Do you intend to keep corals, or just fish?
Lets hope you have a FOWLR system that includes live rock, sand, and a protein skimmer for filtration. This would be a typical, well planned, setup for new marine hobbyist. In this case, I would consider these selections:
Centropyge Angelfish, Basslets, Blennies, Clownfish, Hawkfish, Pseudochromis, Pufferfish from the Canthigaster genus, and Fairy Wrasse. This will give you a good selection of hardy fish that are easily keep in an aquarium with the constraints of a 55 gallon tank.
The most important factor will be the fish that you need to AVOID. A 55 gallon tank is small by marine standards, and you should avoid these fish:
ANY Angelfish that is not from the Centropyge genus, Butterflys, Damsels, Dragonets, Eels, Goatfish, Groupers, Grunts, Hogfish, Lionfish, ANY Pufferfish not from the Canthigaster genus, Rabbitfish, Snappers, Tangs, and Banana Wrasses. Each of these fish present special difficulties in aquariums of your size.
The key to your success will be proper fish selection. Rather than stocking a 55 gallon tank, you should pretend that you are stocking a 29 gallon tank, and simply keep 2 times the fish load. Always ask yourself, would this fish be happy in a 29 gallon aquarium? If not, then don't put it in your 55.