Tetra fish, which are freshwater fish, are a wonderful group to start your tank off with. Freshwater tanks are easier and less expensive to maintain and make great beginner tanks. Freshwater fish can live in small tanks because they are used to lakes and small ponds. If you want saltwater fish, you'd need a much larger tank because they are used to swimming in oceans.
Special thanks to Marc Morrone for showing us the proper way to set up a fish tank.