How do I back up my iMac?

Get an external hard drive and use Time Machine.

see > Back up your Mac with Time Machine - Apple Support

In addition to setting up Time Machine you should enable iCloud storage, get another USB Drive for copies of important files and if you can, get another USB SSD to make a clone of your boot disk. If you can keep the drive with important files off site so much the better

Click ▶︎ Backup with Time Machine.

iCloud is NOT a back up solution as it will not store any settings or Mac OS or applications. For documents it's okay HOWEVER DO NOT rely on it as a backup solution.