If you want to improve significantly, go read someone else’s code and modify it. Try to fix a bug in a program you use, add a feature you want that doesn’t exist already, or even just do something simple for the sake of proving to yourself that you can do it – like compiling it from source and figuring out how to change some small snippet of text in a message box. Even if you don’t succeed, if you put in a serious effort attempting it, you will almost certainly learn a lot from trying.
Edit: changed wording to try to be clearer
GoG homepage > (your name [drop down menu] when logged in) > “Games” > Click on any game in your collection > Download offline backup game installers
You can download installers for whatever systems the game supports – usually that’s just a Windows .EXE installer (+ several .bin files if the game is large). For games intended to run on Linux w/o WINE, you can select “Linux” from a drop down where it says system and it will give you an .sh file.