An existing git repo on your own machine, for example in a directory called myproject. Make sure everything in the repo is committed to a branch.

$ git status
On branch main
nothing to commit, working tree clean

Create an empty repository on github, through the web interface. Get the address of the github repo using the green button. Then back on your own system:

$ git remote add origin https://github.com/youruid/myproject.git

that will tie your existing git repo to the remote. Now you need to pull from github, but no worries, the directory up there is empty, unless you chose to create a README.md file when creating the empty repo.

$ git pull

Now github is ready to recieve an upload of your repo:

$ git push origin main

iff main is the branch you wish to push. You can also use git push --all to push all branches.