Open System Settings
Click your Apple ID name
Select iCloud
Turn on iCloud Drive
Turn on the apps you want to sync to iCloud
Go to Photos
Turn on iCloud Photos if you want photos backed up to iCloud
Go to Messages
Turn on Messages in iCloud if you want messages synced
Go to Desktop & Documents Folders
Turn on Desktop & Documents Folders if you want those files stored in iCloud Drive
Open Time Machine if you want a local backup in addition to iCloud
Use iCloud.com to verify your files are syncing
Check your iCloud storage space
Upgrade your iCloud storage plan if needed