There are a lot of factors to consider when you start a saltwater aquarium. Despite what people think this hobby can be as expensive or as cheap as you like. There are may DIYers out there that use alternative product to achieve the same job.
As a starting point I will list some items you will need, obviously these will vary depending on the size of tank you buy.
Depending on what you would like to keep in your tank live rock is not essential but you will need good filtration if you are not going to use live rock.
1. Glass Tank
2. Heater
2. Thermometer
4. Sea salt ie Red Sea Salt
5. Sand or other substrate
6. Power heads or other device to help circulate water
7. Refractometer or Hydrometer
8. Adequate Lighting - if you plan to eventually get corals research lighting as there is no point in buying twice.
9. RO Water or RO Unit
10. Plenty of time.
This is just a starter to some of the items you will need. There are many more things you may need in the future, such as surface skimmers, protein skimmers, calcium and other supplements. Even Milliput or other reef safe epoxy to help shape and piece your live rock together.
One important thing to remember is that with marine fish they need more room than what you would use for your standard tropical or cold water fish as they generally live in the ocean and have vast amounts of space to move around.