there are many topics to learn as programmer:
the ide
the ui designer
the language (also mainstream c#,asp.net,delphi,swift,java,html,java script,..)
structs & classes
collecting data, text, gps, images, sensors
process data
search data
visualize data, table, charts
file access
database access local & remote
usability
debugging
maths / calculations
input devices
playing sound/music
web api access
logic flowchart / mind maps
artificial intelligence / neural networks
the most fun & challenging is creating any kind of game.