I started with a UI automation scripting language AutoIt (back in ~2005, I was 12 years old then). You can call methods like MouseMove etc. You can also copy files, send keystrokes and bunch of other stuff. (https://autoitscript.com/site/...)
Created an executable using that which will have an icon of a folder named 'Games', which, once opened, will copy itself to system32, add registry entries for launching itself on subsequent startups, would replace windows startup sound file by my favourite song, look for attached USB devices and copy itself to those, if found.
Soon, all my friends' PCs were singing my favourite song on boot.