After logging into GitHub with your new username and password, you will see the following screen:
The home page can be split into several sections:
The Top Menu, which includes a search box that allows various kinds of searches, links to pull requests and issues that are relevant to you, as well as links to browse other people's repositories (marketplace) and tools (explore).
The top menu also includes any notifications you may have, shortcuts to create new repositories and projects, and your account information.
The left side of the page contains a list of repositories you have created or forked, along with a shortcut to create a new repository. It also links to any teams you belong to.
Your own repositories have a 
icon, while those you have forked have a 
icon.
The center and right side of the page contains your feed, showing the activity of any users you are following, and a feed taken from the "explore" link in the top menu, which shows repositories the site thinks you might be interested in.
After you've familiarized yourself with the GitHub website, try creating your first repository (or repo) by following this simple guide:
There is extensive documentation on all aspects of GitHub on the GitHub website.
Some particularly useful guides:
Creating, Cloning, and Archiving Repositories. Includes topics like how to create a new repository from scratch, selecting a license, and cloning an existing repository.
Getting Started with Writing and Formatting on GitHub. Describes how to format text written on the GitHub website. (GitHub uses Markdown.)
Managing Files in a Repository. Describes how to create files, add them to a repository, move them, edit files in your own or another user's repositories, and how to track changes.
Commits. Describes common troubleshooting concerns for commits, including synchronizing between a local repository and the github.com version.
Releases. Creating and managing discrete releases for your project.
Managing Your Work on GitHub. The basics of issues, pull requests, project boards, and milestones, as well as how to find information in a repository.
Collaborating with Issues and Pull Requests. Describes more complex aspects of using issues and pull requests, including how to fork your work, collaborate on code, create review and incorporate changes from pull requests, and fixing merge conflicts.
Administering a Repository. Discusses administrative settings such as visibility, branches and merges, and dependencies.
Understanding How GitHub Uses and Protects Your Data. Discusses privacy and data usage by GitHub.