how can I transfer files from my Mac to the Cloud?

See the information in Set up iCloud Drive - Apple Support

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

Be aware that iCloud keeps a local copy of the document on your Mac. You can access them from the Finder Sidebar or the Finder "Go" menu. Any file you delete on the Mac will also be deleted from iCloud and any other device they are synced with.

You can reduce the storage space needed on the Mac by turning on Apple menu > System Preferences > Apple ID > iCloud (in the Preferences sidebar) > Optimize Mac Storage.