If the first part of the repository doesn’t exactly match your username, it won’t work, so make sure to get it right.
Go to the folder where you want to store your project, and clone the new repository:
~$git clone https://github.com/username/username.github.io
Enter the project folder and add an index.html file:
~$echo "Hello World" > index.html
Add, commit, and push your changes:
~$git add --all
~$git commit -m "Initial commit"
~$git push -u origin master
Fire up a browser and go to http://username.github.io.