iCloud full

I have an iMac 27" computer, an iPhone 15, and an iPad which I don't use often. My iPhone says "Your iCloud storage is full". Many of my document folders on my IMac say "Out of Space." I don't want to use iCloud at all on any of my devices. How do I delete everything in my iCloud (which I don't understand) so I can clear these messages up on both devices. Also, how to I stop anything from my device to go int the iCloud? I guess that question is the same as how do I turn my iCloud off? The reason I ask this is that ihave lots of storage left on both my iMac and iPhone. Please, if yiouo answer, give me step by step instructions because I am not very computer literate. I am running mac OS Sonoma 14.6.1 on my iMac and iOS Version 17.6.1 on my iPhone.

Thanks!