Read books and learn, learn, learn. What to read depends on what you already know and what you would like to know.
If you're interested in getting started with marxist theory in general, https://leftyread.ml/schedules/tilmeeth.html
and the old leftypol list (pic related) are both good lists. For economics and political economy, the other pic related is a good chart. For philosophy in general, and if you're interested in taking the whole historical tour of western philosophy, Plato is not a bad place to start.
Just remember to take it easy and not get overwhelmed by the sheer amount of books to read and things to learn out there. Take it one step at a time, but knowing that reading and learning are really just never-ending processes.