how do I back up my computer on iCloud?

To be clear, iCloud is not for backing up your computer. iCloud is designed to allow access to files and documents from different devices - Macs, iPads & iPhones - and also for file sharing. It is not intended to be a repository for system and data backups for your Mac.

The proper method of backing up your Mac will be to use Time Machine and an external storage drive (hdd or ssd).

Back up your Mac with Time Machine - Apple Support

Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.

It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.

Also see: https://support.apple.com/mac-backup

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

Access and manage files in your iCloud Drive - Apple Support

and

Add your Desktop and Documents files to iCloud Drive - Apple Support