iceblue was talking about a "cycle," which is the Nitrogen Cycle...
You must first have an ammonia source to add to the tank. In the past, a few fish has worked for this. Nowadays, peoplle like a fishless cycle using pure ammonia (no surfectants or other additives) or cycling products, such as BioSpira. Personally, I like to run a new filter on a mature tank for a couple of weeks, which is then removed and placed on the new tank.
After adding ammonia, you'll of course get ammonia readings with your water quality tests (which you should purchase, if you haven't already). Once the required bacteria is in place, they will convert the toxic ammonia to nitrites (also toxic at low concentrations). You'll start to see your ammonia readings drop to zero within a 24-hr period (an ammonia source is still required), while your nitrite readings will rise and eventually spike. More bacteria will then convert this nitrite to nitrate, which is not toxic at low concentrations, though you'll see lethargic and unhappy fish at higher concentrations. Once both your ammonia and nitrite readings are zero after their respective peaks, you can add a few fish at a time until stocking is complete. You don't want to add them at once because you don't want to overwhelm the present bacteria and allow them to build their colonies as you add fish... if you add them all at once, you could see another ammonia/nitrite spike.
Anything else you have questions on?