55 gallons are nice and very versatile. They're quite lovely, you can turn them into quite a productive all fish tank or go 180* and turn it into a reef tank. The biggest decisions are going to be whether you do want to do an all fish or reef tank. Once this you've made that decision, the equiptment will fall into place.
All fish) Lighting : Nothing too much, I recommend a strip, maybe a T5. LIghing for all fish tanks really doesn't become an issue. You can go with the standard flourescent / ballast setup. However, in my opinion going with a T5 or a standard ballast with a 10k or acticnic or both if you get a dual ballast will definately enhance the colorations of your fish. I mean, isn't that why we all switched to salt water anyways?
Filtration: This is soely up to you. Personally I've gotten away with just a canister / hang on filter (Aquaclear something) combination. This worked just fine for my all fish tank. What you have to keep in mind however is that due to the number of fish and the amount of waste they produce, if you do this, very frequent water changes are necessary due to the amount of proteins that begin building up.
Another way that you can go is with a sump style filtration. This is all really dependent on what type of tank you have. My experience with sumps and 55 gallons are quite a pain unless you plan on building yourself a custom stand. The problems lie in the dimensons of the stand itself. Once you get it in there, it isn't going to come out. So if leaking issues ocurr, it becomes a pain. Still, filtration wise you really can't get better than a wet/dry sump. I'd even look into a style of refugium or maybe do it yourself style that incorporates both.
Fish) Again up to you, keep in mind however, choose your fish wisely. Salt is just like freshwater; you can't just go adding the prettiest fish. Kind of put a gameplan, do a little research, even ask the people over there at pet world as to their opinions as to what to start off with, what are hardy, what are well colored. They know their stuff.
Let me know if I missed something or you have a specific question about fish tanks.
Reef) Lighting: The hardest and most complicated aspect to the salt aquarium. This is sometimes the most confusing and most danced around subject about reef keeping. Everyone hast their own opinions; Power Compacts, VHO's, Metal Halides. This is going to be another one of those decisions that you'll have to take a lot of time and look into. Each has it's pros and cons, especially if you use one in combination with another (i.e. Compacts with Halides). Also, light penetration. I'm a nerd, I like formulas and I like being able to calculate what kind of light intenisty and refraction values I'm getting at certain spots of the tank. It can be done, however...it's pretty unneccessary if you just reason through it

. The biggest key is the types of corals you are going to be doing. Soft corals (Mushrooms, Polypes..ect), SPS (Small Polyped Stony Corals) --> these are for the more "experienced" aka however has more lighting and a better established tank. As well as others including the LPS family, Zoanthids...ect the list is practically endless. The decision is for you and from that you can determine what sort of lighting you want. The biggest key here is you don't want to underdo it. Yes you can go overboard and sunburn corals, however i'd rather take that risk than see it bleach out and wither

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Filtration: I honestly wouldn't recommend anything less than doing a wet/dry sump on a 55 gallon or any other reef tank. I'd also recommend a protein skimmer as well, a lot of waste is produced.
Corals) There are endless possibilities however, the more dense you make the enviroment, the more prone your corals are subjugated to being either poisoned or stung by other corals. Corals use many different types of toxins and defense mechanisms to basically lay claim to their territory on the reefs. They reinact this fight in your tank as well. Many times people will load their tank up with a lack of nutrients, filtration, and space and will wonder why their tank crashes and their corals all die. This is a main reason for that, a chemical warfare resides in that seemingly calm water of yours

. Pick and choose carefully. Be particular about the pieces you pick, coral can last you a very long time if well maintained as well as propogate for you. As this is somewhat a beginners guide, I'll leave nutrient supplementation out of it for now, as that will really come later on down the road.
I don't want to overwhelm you, it's really not that different from doing a freshwater tank. You just have to keep a little bit of a closer eye to the water chemistry. Which if you really care about your freshwater tank you will do anyways. Stick with it, if you have anymore questions, really feel free to ask or pop in to PW. They're all good people and all had to start somewhere as well. They'll ahve no problem running you through and actually giving you visuals as to is actually going on.
Hope this helps!
-Chris