You have the tank so that is a start.
Next purchase should be a book. Jay has recommended one, he'll chime in for sure. I can't remember the name, something like saltwater for dummies. Then read it. Maybe read it again.
You will first need a source of reverse osmosis deionized water (RODI). You can buy it or make your own.
You will need salt. Basic instant ocean will work fine. I buy mine on line at Amazon or fosters and smith, always get free shipping.
You will need a refractometer to measure the salinity. Do not get a hydrometer, they are cheap but very inaccurate.
You will need a dedicated container to mix salt water, a food safe 5 gallon bucket will work.
That will get your water right but there is more you need for fish.
A heater, you may already have one from your freshwater tank.
Rock. You can buy all live rock or dry rock. Live rock is expensive and heavy because it is wet, they sell it by the pound. Dry rock is much cheaper and weighs less, also sold by the pound. I like to start with mostly dry rock and add a piece of live to seed it with. For a 55 you probably need 30-40 lb of dry rock and more if you get live.
Sand is optional, just depends on what you like. If you use sand you want argonite sand but not too fine or it will blow around.
That takes care of water and aquascaping.
You will need filtration. The rock is your biological filtration but you probably want a mechanical filter. A basic hang on the back will work fine.
Another important thing is flow. You need a lot of water movement in the tank. For that you will need power heads. One on each end of the tank will work.
You will need a light fixture that is meant for salt water.
Your tank then needs to cycle. This is the time the bacteria are colonizing the rock. It can take any where from 3 weeks to 2 months. You will need test kits to monitor ammonia, nitrite, and nitrate while this is happening.
In the future if you do coral you will need proper lighting. It can be pretty pricy.
You will also want a protein skimmer.
