Note that you can click the "Raw" button on the top-right of this box to see the source code, that is, the original markdown.
The Mastering Markdown page contains all the basic syntax. Read it. Checkout the task lists in that handout; you will be using them. BTW, I have a ton of little markdown booklets similar to this guide in my office. Just ask me for one.
You can add almost any HTML to your markdown and it will work. Useful when you can't get markdown to do what you want.