Hello! First off, welcome to Aquarium Advice!
For a 55g tank, I would recommend a protein skimmer, live rock, and possibly some other form a filtration (like a hang on the back filter).
A protein skimmer takes water from your aquarium, and takes some of the impurities out of it. These impurities are forced into a collection cub that sits on the top of the piece of equipment.
Live rock is actually a form of biological filtration. There are many different kinds available, but you need to make sure that your pieces are free of unwanted pests (I would look up a list of pests that could hitchhike on the live rock online).
For a 55g tank, you could also use a sump. A sump is an aquarium that sits under or near your display tank. The water from your 55g would go down to the sump, and would go through any equipment you have there. In my 125g sump, the water goes through a filter sock to remove debris. In the same area is a protein skimmer that sits in the sump (there are skimmers that will hang on the back of the tank, and in-sump skimmers). Also, in my sump I have a refugium (well, I have a spot for it, it is not setup yet). A refugium is a place where you can keep more live rock, sand, and macroalgae (the good kind algae for saltwater aquariums). The macroalgae is supposed to filter the water somewhat. A refugium is also a good spot for different kinds of pods
(i.e. amphipods, copepods) to reproduce and build up a population. Many fish will eat these little creatures.
The last area of my sump contains only a return pump to bring the water back to the tank.
The protein skimmer and filter sock are in the first "chamber", the refugium is in the second chamber, and the return pump is in the third. Also, when the water goes through the second to the third chamber, there is a block of material that I believe is designed from getting microbubbles pumped back into your aquarium. Each chamber is really a different section, so I have there different divided sections in my sump (the water passes through them all)
You will also need a heater, I recommend two because if one breaks, you have a back up. If you think your tank will get very hot with your lights and/or you live in a hot climate without air conditioning you could consider a chiller. There are, however, very expensive!
If you have any further questions just ask!