Little tank means a little change can be huge in terms of effect.
A 30 gallon is a good medium size---big tank solidity, but not so cranky as a 10, and it will still run on a Penguin filter and adequate lighting if you want to get into the simplest kind of saltwater.
To do the modern way, you want a reef-plumbed tank, a skimmer, a sump, a pretty potent return pump, good lights, live rock if not live sand, and a flock of test kits.
You say you want to keep corals: you can, but corals come in several different levels of what-they-need. Simplest, the zooanthids and mushrooms, many of which will flourish under lower light and basic equipment; a 30 with a Penguin can do those.
Next in requirements: with better lighting, and the sump/skimmer/live rock method, you can keep the large polyp corals like frogspawn, bubble, and others of that type: some of them will eat most anything you give them.
The highest requirements, definitely mh lighting and potent skimmer and sump, the sps, the branching colorful sticks and their type...they're very do-able if you have the right light and a good amount of flow and the right test kits (cal, alk, and mg). You can get sucked further and further into that realm of calcium-hungry creatures that get you into calcium reactors and other automations that can get pricey.
I'd say if you want to start, start with a tank that can become the sump of a bigger tank, because people who get drawn into this hobby end up getting bigger tanks---and get your feet wet, so to speak.

Get mushrooms and the like, trade with friends in the hobby, learn to reproduce those and get expert with the equipment and the water chemistry. You can have your mushrooms, your shrimp, a few nano-fish, and have a lively, colorful tank. Don't start with an anemone, if you'll take my advice: everyone who's seen Nemo wants to try, and they're a special kind of problem. If you want a good-looking tank, and corals call to you, try the mushrooms for a start: they stay where you put them (mostly), they reproduce easily, and there are tons of varieties that will get you started. You can parlay them into a few tolerant stony corals and better lighting, and you'll do fine.