Single Repo
If you need to quickly set the user and email address for a single git repo, you can do this:
git config user.email john.doe@example.com
git config user.name "John Doe"
If you want to modify the .git/config file directly, you can add in this block:
[user]
name = John Doe
email = john.doe@example.com
All Repos
git config --global user.email john.doe@example.com
git config --global user.name "John Doe"