I created the private repo examplesite/myprivaterepo using the Github UI from my browser.
Then I went to my go directory (on the desktop) and cloned it:
If you just want go get to work real fast, and move along with your work...
Just export GIT_TERMINAL_PROMPT=1
$ export GIT_TERMINAL_PROMPT=1
$ go get [whatever]
It will now prompt you for a user/pass for the rest of your shell session. Put this in your .profile or setup git as above for a more permanent solution.