Hello Pm...
You can add a few fish any time. It's the fish waste that starts the "Nitrogen Cycle". You should use hardy fish. Platys, Guppies, White Clouds and Zebra Danios are good for a smaller tank.
You must have a reliable water testing kit handy, because you'll need to test the water daily for traces of ammonia and nitrite. When you have a positive test result, then you must remove a quarter of the tank water and replace that with pure, treated tap water.
When you have several daily tests that read "0" for the above toxins, then your tank is cycled.
I put stem plants in the tank to help with the cycle. Anacharis and Pennywort worked well.
B