My husband is into saltwater tanks, so I'll see if he can reply to this thread since he's more knowledgable about the topic than I am.
We've had saltwater tanks for years, and started with nano tanks. We've had a 90 gallon set up for about 3 years. Generally, the smaller the tank, the harder it is because there's such little room for error. If you don't catch a problem right when it starts, the whole tank can crash.
I'd actually recommend getting a new tank that is meant for saltwater, like a Nano Cube or Bio Cube. They're all in one tanks, which means that you'll pretty much just need to add live rock and saltwater. After you add the live rock (your biological filtration) and salt water, you'll have to let the tank "cycle" for a while. If you don't, adding livestock will cause an ammonia spike and kill all of the fish.
With a 15 gallon tank, realistically you can't have more than 2 or 3 fish. I'd recommend something like 2 clown fish and a cleaner shrimp. Make sure to get "clean up" crew, which are snails and hermit crabs for the most part.
Are you planning on having coral? The all-in-one tanks have lighting built in which is strong enough for most soft corals and even some LPS (long polyp stony).
I also agree that nano reef and reef central are great resources. Check out your city's local section on reef central- people get in and out of the hobby all of the time, and you can usually score tanks, equipment, and livestock for good deals