First, read here on the Nitrogen cycle. It takes longer than a week, and you have to test more than just Ammonia to make sure it's done:
http://www.wetwebmedia.com/FWSubWebIndex/fwestcycling.htm
This is the most important, and most often skipped, part of starting a tank. It's absolutely crucial!
Secondly, understand that the 'Common' pleco many pet stores will sell you can grow over a foot long! Not only that, but they poop, poop, poop, and they can quickly foul water quality in such a small volume.
The Corydoras catfish may be a better bet for you. They're armored catfishes which stay small, and are schooling, so you would want a group of six for a 20 gallon tank. There are many different varieties. They are generally very active, in good, clean, warm water, and venture all over the tank.
As for other stocking options, some fish to avoid, first:
--The Dwarf Gourami -- the majority of the ones brought into the U.S. are sick with some Gourami-specific indovirus, and die shortly after purchase.
--The Molly -- will be suggested as a community fish, but is actually a bad one -- --Mollies need hard, alkaline water, and this doesn't mix with so many community fish. They are actually better kept as Brackish fish.
--Goldfish -- they grow large and are messy, requiring robust filtration.
--Bala Sharks, Rainbow Sharks, Iridescent Sharks -- all of these minnow-sharks grow fairly large (some grow very large -- think three or four feet!) and become gradually more aggressive as they age. They're not good community fish, and would quickly outgrow your tank.
--Most Cichlids -- most cichlids found at the chain stores -- Blood Parrots, Africans, Oscars, etc. are going to outgrow the tank you've got available and not be good community fish. There are some, such as Rams, that might work, if you find them -- a pair would probably be quite happy in a 20 gallon.
--Pacu -- don't be one of those folks. Please.
--Those cute little frogs they have -- require specific needs and usually don't mix well in community aquaria. The same goes for those cute little crabs!
Some good choices:
--Any of the smaller tetras, such as Neons, Black Neons, Glo-Lights, or Cardinals would work well in a tank this size. Their needs in terms of water chemistry also match the needs of the Corydoras mentioned above. All tetras are schoolers; buy a group of six or more. Most stay in the middle of the water column. Watch out for the larger tetras, especially Black Skirts and the Albino morphs of the same -- they can be nippy, and actually grow pretty big, for a little tetra!
--Hatchetfish -- a type of tetra (characins). Again, a schooler, but a top-level fish. Jumpers, so keep the tank tightly covered, but very active fish which will provide interest on the top level of your tank. Again, these guys have similar needs in terms of water chemistry and temperature as the Cories and other tetras.
--Guppies -- if you want to go the livebearer route, guppies are a good option. These, though, will not mix with the above fish well, as they require harder water in order to thrive. Make sure and purchase at least two females for every male because males are constantly wanting to mate and will stress the females.
--Danios -- are a great choice. A schooling fish, they like slightly cooler water than the fish listed above.
So, read up on each and every species before you buy, not once you get them home. Make sure they're compatible in terms of temperature, pH and KH, and personality/temperament. Read, read, read!
Don't hesitate to ask if you have questions!
Edit: My God this might be my longest post ever.
