These are all articles that are most helpful before fish are added to the aquarium.
The stage between buying the equipment and adding fish can be very short. The first step is to get everything done discussed in the Before You Fill the Tank article. Then get everything setup and running. Make sure all pumps and filters are working. Check for leaks. Allow the heater to bring the tank up to the proper temperature. Now is the time to get the substrate and all decorations into the tank. Make sure the temperature is correct and stable. Test pH, nitrate, etc.
Before fish are added the tank must be cycled. For a complete explanation of good bacteria and cycling, please read: Cycling and Understanding the Good Bacteria in Aquariums.