Star wars politics ques.?

watching star wars series agian but am very confused by the alliance politics and which side is the bad and which is good and why they keep changing? I'm even confused as to if "the Empire" is the good guys or the bad guys. then they throw out the terms rebel alliance, federation, and sepertisis. can someone please explain to me whos who and which fractions are good and which are bad. (i know luke is good and vader is bad but they throw all these alliance goups names out there). thanks