Anons,
If you are new to Ghidra (like me) and are looking for simple programs to reverse-engineer/crack, check this site out:
https://crackmes.one/
And for a tutorial on how to perform some really simple cracking, look:
https://www.youtube.com/watch?v=fTGTnrgjuGA
Try to practice caution and always reverse engineer through a non-networked Virtual Machine. If you need a free-and-easy VM check out VirtualBox:
https://www.virtualbox.org/
You will need an operating system to install on it, and unless you have an extra version of Windows or Mac laying around you can grab a free Linux distro from a variety of places. One of such being Ubuntu:
https://ubuntu.com/
Good luck!