Everything I’ve read or seen on places such as Youtube videos say that the first and best upgrade for an upgrade to one’s audio system is a cartridge upgrade.  I have a Technics SL-1900 that I have had for decades.  Yes, decades.  I got it in either 1980 or 1981.  
I can not determine which cartridge it has now.  But what would be a good upgrade to consider?  I know this is a silly question with the 1000’s of possibilities.  But what are a few starter considerations?