So you have a working repo at Bitbucket and want to sync it to Github? I will explain how to achieve this.
# Clone the Bitbucket repository to your local machine:
git clone --mirror ssh://git@bitbucket.org:your_name/your_repo.git
cd your_repo.git
# Sync from remote (prune is to delete the local branches that are no longer on the Bitbucket repository)
git remote update --prune
# Push latest changes to Github
git push --mirror git@github.com:your_name/your_repo.git