Hi Ania, thanks for your question.
In the Polish language, "się" is a reflexive pronoun used in various contexts. Here are some common ways "się" is used:
Reflexive Verbs: "Się" is used with reflexive verbs to indicate that the subject of the sentence is also the recipient of the action. For example:
Myję się (I am washing myself).
Ubieram się (I am getting dressed).
Reciprocal Actions: "Się" can be used in sentences involving reciprocal actions between two or more people. For example:
Spotykamy się (We meet each other).
Passive Voice: In the passive voice, "się" is used to indicate the recipient of the action. For example:
Drzwi otworzyły się (The doors opened).
Impersonal Expressions: "Się" is used in impersonal expressions to convey a general or unspecified subject. For example:
Nudzi mi się (I am bored).
Śniło mi się (I dreamt).
I hope this helps. I would not worry about it too much, try to use the langauge as much as possible, try to notice the usage and slowly incorporate it yourself.