How do I back up my new computer from my iCloud

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

If you had some or all iCloud syncing options turned on in your old laptop, you can turn the same options on in get new one (using the same Apple ID) and the data will sync with it. See the following for more information about syncing.

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 the Apple article: How to back up your Mac - Official Apple Support

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

Access and manage your files in iCloud Drive - Apple Support

and

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

icloud is not a backup option for Macs.

That said, if you had documents and desktop in icloud enabled, you should be able to just sign into your new Mac with your icloud account and sync all of your documents down to the new one. You can re-download any apps purchased from the App Store at no charge. If you had iCloud Photos enabled, you can also re-sync all of your photos.

Anything that was not synced to the cloud or backed up on media or a cloud service you have access to is gone.