It depends on what you want to use the variables for -- like storing numbers and persistent data or music and images. Depending on what you want the variables to do, there's different ways to implement them in Ren'py.
I read forums and stuff to see if there were certain things I could do but I started with watching the videos in these playlists:
Visual Novel Design's "Ren'Py Essentials":
https://youtube.com/playlist?list=PLMdFGlfdL57MSSForRZZBJtS-N9thXSA6&si=keI05hqF...
Zeil Learnings's "Visual Novel using Ren'Py":
https://youtube.com/playlist?list=PLlZCSNfSKux2hXoo46rbBIVzl-7pZFRQh&si=F4tSIpid...
Learning the basics makes it easier to see what the code in mine and others' Ren'Py games is trying to do. I hope that helps! Good luck!