Agitprop is a very broad term. What are you meaning exactly? What is the target media? Screen, print, posters, publications, clothes, billboards, TV,.... Different tools.

Do you have ideas about what and how you want to communicate and how but lacking the technical skills? This is more important and harder to learn. Simple free tools can be used to excellent effect without much tech skill.

I learned on adobe a long time ago and it was great. Honestly gimp is OK for some basic stuff but I wouldn't want to be doing anything serious in it. Inkscape continues to suck ass; been hoping it would improve since the killustrator days but am not optimistic.

But gimp/Photoshop/krita illustrator/inkscape InDesign/scribus/libreoffice... These are 3 different kinds of tools for 3 different purposes. So you need to think about your goals first.

Shorter answer: The way you learn is to spend hundreds of hours fucking around. Think of something to do, can be anything random, start with whatever program you feel like and try to do it. Then go on the internet to solve problems you have. Peruse the software documentation and tutorials to get a feel for what kind of task your software excels in and switch if needed. Learn about layers at the very first opportunity.

I rec the site alternativeto.com to find software. Type what you know (e.g. Photoshop) into the search and then use the filters.