If you use Kerberos and have a trust relationship with the Active Directory domain that your TFS server is on, you need not enter any password, git-tf will use your Kerberos ticket to authenticate. This, of course, will only work with on-premises servers; it will not work with Team Foundation Service.
To cache your username and password for the repository, you can set these in your .git/config
:
git config git-tf.server.username myusername
git config git-tf.server.password mypassword
However, do note that this will store your password in plain text, which is why Kerberos is preferred. I am interested in adding support for the git credential storage mechanisms, but this does not yet exist.