Welcome to Aquaria Central!
One of the more common questions that will be asked is "what kind of tank do you want to setup?"
There are several different kinds of freshwater tanks... you have your fish only kinds, fish with plants, and (yes) plants with fish. On top of that, different kinds of fish can dictate how a tank should also be setup.
What I'm outlining for you is how to setup a fish only kind of tank. This outline can be easily modified for setting up other kinds of freshwater tanks.
1.) Go to your local grocery store, and get yourself some pure ammonia. It should be under the cleaning section.
2.) Get yourself some ammonia and nitrite testers. Nitrates are nice, but definately not absolutely needed.
3.) Put in your tank substrate (ie. aquarium gravel, sand, etc.), and put in your tank decorations if you have any.
4.) Fill your tank with dechlorinated water. Wait a day for the dust in your tank to settle down. Pre-wash your substrate and/or decorations to minimize this.
5.) Start up your filters and air pumps (if you have any).
6.) Add enough ammonia to achieve a concentration of 3-5ppm in your tank. You can figure out the concentration of ammonia by using your ammonia tester.
7.) Monitor your ammonia levels daily. If ammonia levels drop below 3-5ppm, add more. Try to keep your ammonia levels as level as possible.
8.) After awhile you should notice your ammonia levels start dropping rapidly, this is a good sign. The ammonia consuming bacteria in your tank are starting to get well established. When you notice ammonia levels constantly hit zero after 24 hours, start using your nitrite tester in conjunction with the ammonia tester.
9.) From this point on, monitor both your ammonia AND nitrite levels. Continue to add ammonia everyday (to keep the 3-5ppm reading). After awhile, you will notice nitrite levels dropping just like the ammonia levels did. After a few weeks, nitrite levels should start hitting zero after 24 hours on a consistant basis.
10.) Once you notice ammonia and nitrite levels constantly hit zero after 24 hours for a few days, congradulations, your tank has cycled! Do a large 80-90% water change to reduce nitrates in your tank, then within a day, add your fish.
11.) After adding your fish, monitor ammonia and nitrite levels for a few days to see if any thing shows up. If you do happen to detect a small amount of ammonia and/or nitrite, do water changes to keep those levels as low as possible.
After successfully cycling a tank, it should remain stable as long you as maintain the tank properly. This includes doing weekly water changes (I suggest at least 30-40% at least once a week), and good gravel vacuuming whenever you do a water change. Keep in mind that as long as the water going into your tank is dechlorinated, and has simliar paramters of that of your tank water (ie. pH, temperature, hardness), you can do large water changes without consequences. Every now and then, I do two 50% water changes on my fish tanks on top of my weekly 50% water change. This in essence more or less eliminates most things floating around in my tank... I've been doing this for years, and my fish are thriving. Doing a good amount of water changes also insures that your tank water is similiar to that of your source water (ie. tap water, well water, etc.). Since your tank water is silmiar to your source water, you can easily do very large water changes in case of an emergency (ie. dropping a can of food into the water) with a problem.
HTH
-Richer