What's the difference between 저 and 제가?
Not sure why all the downvotes for comments herein. People's advice is vague because particles in Korean are unbelievably hard to get right, let alone explain.
I don't fully understand the issue myself Ciro, but from experience it seems like 저는 or 제가 would both make "I" the main, active agent in a sentence. The difference of 은/는 and 이/가 is the most difficult aspect here, but I believe 는 does extra work to either single out the noun as some very important detail, or introduce it for the first time in a conversation.
In practice you shouldn't worry too much about getting these wrong as a learner IMO, Koreans appreciate what a difficult part of their language this is.
Just as an aside, a third version 저를 would have you as the object of the sentence, the thing being acted upon.
https://www.youtube.com/watch?v=fCxLNRLntc0 저는 and 제가 both mean "I". When to use a "topic" particle as opposed to a "subjet" particle can be confusing. I think it takes most of us a while to truly gain an understanding of the different nuances, but this video gives a pretty thorough explanation.