Law / Taxes when teaching online while living in Poland [3]

Cześć everyone and thanks in advance for your help.
I'm a European citizen and have been living in Poland for 2 years. I'm a language teacher for a local school but recently I've also started giving private online classes on websites such as preply and italki, for those of you who know them. They're websites where you create a profile, students from around the world find you and the lessons are made on the platform (and the platform keeps a percentage of what the student has paid you).

Since I'm starting to work more and more with these, I was wondering, how does it work with taxes? How should I declare these earnings? And if I stopped working for my school and did this full time, what legal procedure should I follow?

Dziękuję bardzo