Is there a marine newbie FAQ somewhere that's meant for people who already know freshwater? All the ones I've seen have things like "you'll need a tank and a stand."
Most make 75-125 gallons per day, quite a bit slower then your faucet, also cold water is optimal for filtration.
For decor you can do plants, and or fake corals.
Adding lighting will do it and a skimmer if you didn't already have one as a FOWLR, may also have to up your circulation. You'll also have to start monitoring other parameters such as calcium, stronium, etc.
I've never felt the need to add anything in terms of decoration, but you can. There are also marine themed decorations that could be used. I wouldn't--I'd rather put more live rock in than take up space with something that's not serving a purpose.