Hi Ania, thanks for your question. First you need to edit your account and put the information about the languages you speak and want to learn. If you tap your picture or avatar in the left top side of the screen and tap Languages there, you will be able to do so.
I think that based on this information Tandem will match relevant people. In your case, Polish language speakers who want to learn English.
You tap on profil pages to read more about the person, their interests, etc.
If you like somebody, send them a message asking if they want to learn together.
Many people use just chat to write things and get them corrected but I prefer to arrange online lesson on Skype, Zoom or even on Tandem and speak in real time.
Please let me know if you have any additional questions.