Pushing needs an remote repository.
pi@retropie:~/RetroPie-Setup $ git remote -v
# output
cmitu git@github.com:cmitu/RetroPie-Setup.git (fetch)
cmitu git@github.com:cmitu/RetroPie-Setup.git (push)
origin https://github.com/retropie/RetroPie-Setup (fetch)
origin https://github.com/retropie/RetroPie-Setup (push)
If you haven't added your own repository as a remote, add it first (make sure you set-up SSH keys authentication for the repository, Github doesn't allow pushing over HTTPS anymore with user/pass). Then, when you create a branch and want to push it to your repository, just
git push --set-upstream mine test
where mine is the name of your fork's remote repository.
pi@retropie:~/RetroPie-Setup $ git push --set-upstream cmitu amiberry-5.2
Enumerating objects: 45, done.
Counting objects: 100% (43/43), done.
Delta compression using up to 4 threads
Compressing objects: 100% (5/5), done.
Writing objects: 100% (5/5), 871 bytes | 37.00 KiB/s, done.
Total 5 (delta 4), reused 0 (delta 0), pack-reused 0
remote: Resolving deltas: 100% (4/4), completed with 4 local objects.
remote:
remote: Create a pull request for 'amiberry-5.2' on GitHub by visiting:
remote: https://github.com/cmitu/RetroPie-Setup/pull/new/amiberry-5.2
remote:
To github.com:cmitu/RetroPie-Setup.git
* [new branch] amiberry-5.2 -> amiberry-5.2
Branch 'amiberry-5.2' set up to track remote branch 'amiberry-5.2' from 'cmitu'.