backup my pc onto icloud

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

It is possible to sync data from a Mac (or from a Windows PC using iCloud for Windows Set up iCloud on your Windows PC - Apple Support ) to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive). 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 computer 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.

See more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

Access and manage files in your iCloud Drive - Apple Support