Ok this is my opinion. First 20 g will fill up quickly so be very picky about what corals you get. Do not get quick growing corals unless that is the only coral you want in the tank because they will take over the tank also being a small tank you will need to keep a very close eye on your parameters.
Second. You have to use ro di water. Either find a reliable trustworthy place to buy the water or get your own ro di unit.
Then you will of course need salt (I like reef crystals), a skimmer ( you can get hang on the back skimmers), heater, power heads, good lights, live rock, a hydrometer, and test kits for ammonia, nitrite, and nitrate in the beginning. You may meed other test kits the more advanced you get.
This is iust the very basics. Hope I didnt forget anything. Saltwater is way different than fresh
But it is more rewarding imo