If you have cloned a fork (e.g. an open source project on Github) you may not have push access to the upstream repository, so you need both your fork but be able to fetch the upstream repository.
First check the remote names:
$ git remote -v origin https://github.com/myusername/repo.git (fetch) origin https://github.com/myusername/repo.git (push) upstream # this line may or may not be here
upstream is there already (it is on some Git versions) you need to set the URL (currently it's empty):
$ git remote set-url upstream https://github.com/projectusername/repo.git
If the upstream is not there, or if you also want to add a friend/colleague's fork (currently they do not exist):
$ git remote add upstream https://github.com/projectusername/repo.git $ git remote add dave https://github.com/dave/repo.git