First of all, welcome to the community! Secondly, it this might have been a better place to post: https://github.community/t5/Pages/bd-p/pages, since I think the feature you are referring to in your post is Github Pages. Anyway, I’ll try to answer your question.
You have to types of GH Pages sites: an orginization or user site - this is a website for your whole profile and a repository stie - this is a site intended for use by just one repository.
If you want to create an orginization/user site just create a repository named yourUserName.github.io. Then, follow the tutorials on how to set up a GH Pages site in that repository.
If you want to create a site for a single repository, just create a branch named “gh-pages” and put your site in there, according to the tutorials.
If you want to access your user/orginazation site just head over to yourUserName.github.io. If you want to access a repository site just head over to yourUserName.github.io/respositoryName!
You CAN youse a custom domain if you want to, but it is not mandatory.
The following link acutally contains all info on what I just said: https://help.github.com/articles/user-organization-and-project-pages/.
I hope I anwered your question.