If you get this error when syncing (pushing) to your remote origin:
git fatal: no upstream configured for branch 'master'
This usually means you have missed some commits and need to get them in sync first before you can commit your new changes too.
you will need to open git to the folder where the repo is hosted and enter in:
git pull origin master
This will pull down any changes you have missed to the master branch (You can replace master with the branch you are working on)
Then you can merge any diffs and commit to your local git, then a sync to your origin should work.