Buy at least a 15 gallon tank (2x1x1) if you can afford it. Despite marketing to the contrary, small tanks are not 'ideal for the beginner'. A large tank is more stable in terms of temperature and water chemistry. Wash filter sponges and other 'biological' media in tank water, NOT tap water, to avoid killing beneficial bacteria.
Carry out water changes regularly - 25% every 15 days should probably be considered a MINIMUM for an average community tank.
Do not overfeed. Feed small quantities of food and watch fish eat it before adding more. Do not be tempted to feed fish because they 'look hungry' - 2 times daily is sufficient.
Do check the requirements and compatibility of species BEFORE buying. Observe fish carefully before buying, avoid any with split fins, damaged gills, etc. Release new fish into the tank gradually - float the bag for at least 15 minutes to equalize temperature. Use a quarantine tank for new fish wherever possible. Read up on the 'cycling' process and its consequences. Stock slowly to give the bacterial population time to increase to match the additional waste load.
Do not overstock. 1" of fish per gallon is often used as a general guideline, but this cannot be used as a hard and fast rule - you can't put a 10" fish in a 10 gallon tank. Remember that your tank will be easier to keep healthy if you understock.
Avoid the use of too many chemical additives unless you fully understand the consequences of their use.