Craft database and Github - using multiple computers. Issue
It's a good practice to designate one instance on one computer (or server) as the "canonical" instance. You can make all database changes there, adding fields, etc. It's easy to download that database via the Backup Database tool in the admin panel. You can then import that dump file into databases on other machines as needed with a tool like SequelPro (Mac).

Some prefer to include an updated database dump file in the git repo every time a change is made but that can get pretty tricky.