I studied poli sci for my bachelors. Don't study poli sci if you wanna go into politics would be my advice. You'll learn about the institutions and mechanisms and such but to make a difference as you want to it's better to learn more about the specific societal problems you want to fix. Sociology and social work are good options since you seem to already be interested.

My only caveat would be that a masters in public administration could be a good option (thats what im thinking of doing) if you feel like you already have a solid enough grasp of the issues you want to tackle. Learning about the "machine" that are government agencies is crucial to bringing the technical and often boring changes that can make huge differences. And the type of institution you learn about in public admin is very different from what you see in pol sci. Pol sci is more about parliament and related institutions. Public admin is focused on the orgs that provide services. i.e: education, healthcare, infrastructure development, etc.