How to turn off iCloud syncing

I made the mistake of accidentally turning on iCloud syncing from my iMac. Now it has uploaded all my photos to the cloud as well as everything on my desktop and all my documents. I am constantly getting alerts that I am almost out of space and need to buy more storage. When I try to delete a file or photo from the iCloud I get dire warnings that it will be deleted from all my devices. I want to delete them from the cloud but keep them on my iMac. How do I do that? And how do I make the syncing stop?