saltwater are usually a bit more indepth, in terms of research and maintence. If you plan on going with a freshwater tank and want several fishes in your 10/20 gallon, i would look into tetras, guppies, neons, zebra danios, barbs, etc. These fish are schooling fish, meaning they prefer to be in groups of 4 or more. Tiger barbs are well-known for nip-picking on the fins of other fish. As far as maintence, you'd need to learn about the nitrogen cycle first. Start off with an empty fish tank with detoxified ammonia(your piss is considered ammonia and works fine). Your filter, bio wheel prefered, will collect this ammonio from your tank and be fed to the bacteria in your bio-wheel. Once you've accumalted enough bacteria in your tank, nitrogen and nitrates are low, you can safely begin putting fish inside your tank. The key to freshwater tanks is to have enough bacteria in your filters, whether its bio wheel, canister, wet/dry sumps. Once you have that bacteria established in your filter media, then these can absorb the ammonia/nitrites coming from the "poop" of your fish and break these toxics into a lower form, known as nitrates. This is why they have freshwater test kits, to monitor your readings. As far as a heater goes, i think all aquariums are required to have a heater, even if your only carrying goldfish. The reason is to keep the tank at a stable temperature all the way through. Having no heater will cause fluctuations in temperature. Fluctuations at the slightest bit will cause stress to your fish, give diseases such as "ick" to your fish, etc.
