If you want to publish the site at http://USERNAME.github.io, you need to create a repository (from your GitHub account)
called USERNAME.github.io, in my case mcaliman.github.io.
GITHUB_ACCOUNT=USERNAME
cd ${GITHUB_ACCOUNT}.github.io
git init
git add .
git commit -m "Initial commit"
git remote add origin git@github.com:${GITHUB_ACCOUNT}/${GITHUB_ACCOUNT}.github.io.git
git push origin master