Terminology
Markdown
Markdown is a simple WYSISYG text formatting. It allows you to focus more on what you're saying rather than how it looks.
Here are some helpful guides:
README.md
The contents of your README.md file is shown at the bottom of the repository front page. This is a very good place to describe your project. The .md extension stands for Markdown (see above). This is very easy to edit using the online editing tools.
Issues
To record an issue with a project, click on the Issues link on the sidebar of the repository.
Then click on New Issue
Fill out the details of your issue. Include as much information as possible including tagging people for help with an "@" symbol, referencing other issues with a "#" sign, and selecting labels on the right.
Here is what it looks like once an issue is resolved
Advanced GitHub Concepts
Here are some resources that will explain advanced GitHub topics in more detail
Step-by-step course for the complete beginner
Getting started with Git and GitHub 
GitHub Glossary 
Git Cheat Sheet
More than you ever wanted to know about Git 