When dealing with nano tanks, light stocking is key to your success. Especially for beginners.
Although you didn't exactly ask for this advice... a 20g tank is not the size I'd recommend for beginners. I'd start with 55g. 75g is even better as setting up your rocks is much easier in a 75.
Regardless though... in my 24g nano tank I have a maroon clown, a yellow watchman goby, and a firefish goby. For inverts I have a small bubble tip anemone (certainly not for beginners, FYI), a handful of hermit crabs, snails, and a pistol shrimp. I had a cleaner shrimp, but it died. For corals, I have all soft corals. Some leather frags, some green star polyps, mushrooms, etc.