Doubt it matters that much what software you use to do some basic things - you could even use something like LibreOffice Writer if you really wanted to. I'd imagine learning some of the theory behind making eye-catching, readable works that effectively communicate something matters much more. Of course there's a lot more artistic things you can do to improve things.

Like ReadFanon said, I've heard good things about Krita from artsy people if you want something free. Clip Studio Paint seems like a pretty standard recommendation for paid software for individuals. PS seems to only be something people use because they learned to use it like a decade ago, so its easier to keep using it than to learn new software. GIMP seems pretty universally disliked (granted, its what I learned to use for some very basic things over a decade ago and I rarely use art programs, so its good enough on the rare occasions I'm trying to do something that requires one). Not sure about specifically for agitprop though.

I don't do design or art though, so probably better to learn from other people. Some artists love explaining their processes while they work, so just chillin' with them can be a way to get some basic understanding how workflows and how to use software, but so many artists have their own way of doing things, so I'd say just try things and see what does and what doesn't work and don't expect things to work well on your first try.