Take every opportunity to gain background knowledge about the aviation industry. Airports, airlines, aircraft and how it all fits together.
Browse the aviation regulatory websites, this website, wikipedia, aviation magazines, library books, aircraft accident reports. These will give you a good grounding in the language of aviation as well as some of the many industry abbreviations.
In my view this will give you a good foundation on which to build the specific ATC knowledge which is best acquired from good training organisation.
Helen