Once you've got your tank set up and the filter running, the next step is to make your way to the local fish store to buy your first fish (or two).
If we define a good beginner's fish as one that is easy to feed and care for, hardy, able to live in a variety of water conditions, and attractive, then there are a number of widely available fishes which fit the bill nicely. Many of these are regularly sold as beginner's fish. But watch out! Many of the fish sold as beginner's fish really are not well suited to that role.
Many of the smaller schooling fish make ideal first fish. These include White Cloud Mountain Minnows, the several commonly available species of Danios and Rasboras, and most available species of Barbs. For those with a slightly larger tank, Rainbowfish make a great schooling fish. Corydoras Catfish is ever-popular schooling catfish.
While many beginners are tempted to get just one or two of each of several different schooling fish, this should be resisted. Schooling fish do better if there is several of their own species present for them to interact with. A minimum of six of each of the midwater schooling fish is recommended, while four is the bare minimum for Corydoras. In the long run, a school of a dozen fish showing their natural behavior will be more pleasing than a mixed group of fishes unhappily forced to share the same tank.
The easiest answer to that question is "one fish at a time." As far as how many in total can safely survive, a frequently used rule-of-thumb is "up to a maximum of 1 inch of fish per gallon." Much discussion of this rule has suggested that it really should read, "up to a maximum of 1 inch of slim bodied fish per gallon."
Slim-bodied could be fish such as Neon Tetras, White Cloud Mountain Minnows, Danios, etc.; medium bodied might be Red-tailed Black Sharks, Tiger Barbs, Platys, Corydora Cats, etc. and heavy bodied would be Goldfish, Oscars, etc.
In other words, this is only a rule of thumb, and the maximum population that is safe and humane will vary from tank to tank. Factors that increase your possible fish load include:
Likewise, factors that decrease your possible load include:
We are accustomed to thinking of bacteria reproduction as "explosive". Many bacteria can double their population size in hours. However, the appropriate nitrifying bacteria are relatively slow to reproduce. There will be a time delay between the increased waste production of additional fish, and increased waste processing by the bacteria. In extreme cases, the ammonia increase could harm or kill your fish before the bacteria population had time to "catch up" to the amount of available nitrogenous wastes.
This is why it is wise to add fish slowly and gradually. Safely bringing your tank's population up to the maximum load can take more than 6 months. In fact, it should be permitted to take at least that long. Leave breaking the rules to those with more years experience than they have fish.
Once you get the fish home you should set the bag (store keepers put fishes in plastic bags filled with water) in your tank, allowing the temperature to equalize. After about a half an hour or so, add a 1/4thcup of tank water to the bag. Repeat this process once every 15 minutes for an hour, removing any water if the bag gets too full. Any water you remove from the bag should be disposed of. It will most likely contain parasites and faeces.
After you have the fish acclimated to your tank's water chemistry, there are a couple of things you can do. You can place the fish directly into the main tank and hope for the best, or you could place the fish into a quarantine tank. In either case, quickly net the fish from the bag to the tank so that no store water gets transferred to the tank.
The best scenario is to place the fish in quarantine. Keep the fish in the quarantine tank for 2 weeks and watch for signs of disease. If the fish gets sick, you can medicate the quarantine tank without affecting the chemistry of the main tank. If you are going to quarantine the fish, you should acclimate the fish to the quarantine tank's chemistry, not the main tank.
While a quarantine tank is a good idea, it is most likely that you do not have such a luxury. In this case, be extra careful to select healthy fish at the store, and carefully monitor your new arrivals for the first few weeks in your tank for signs of stress and disease. You always risk infecting the other fish in your tank when skipping quarantine procedures.