About noise during development.

How to get the cleanest image as possible during development?

I know that the slower I agitate the film during develop the less grainy the image become. But what else has influence on it?

Choice of film and developer and attention to tight control of exposure, development time and temperature control throughout the entire cycle.

You need to test to find your optimum film speed and development time for normal subjects/lighting. That is not the same as the manufacturers recommendations because meters, shutter speeds, thermometers even timers vary. The variations might cancel each other out or compound huge variations, only testing tells.

Ian