Before you start
Water first. Fish second.
What cycling a tank means, and why testing matters.
A biological system
The filter hosts bacteria that turn ammonia into nitrite and then nitrate. Clear water doesn't prove that process is established.
Test and record
Write down your water test results and dates. Don't decide a tank is ready just by counting days. The My aquarium diary keeps your notes.
Add fish with a plan
Before choosing fish, check that the system is stable and what each species needs. The linked article explains the cycle; don't treat a number of liters as an automatic stocking rule.
Source: Aqueon · The Nitrogen Cycle