The only markdown you really need to know.
Markdown is a great way to format text inline, without getting in your way. i.e. it’s not ugly and cumbersome like HTML tags, and your hands don’t have to leave the keyboard to access formatting toolbars like they might in a traditional text editor. Personally, markdown helps keep me “in the zone” when I’m writing. That’s why this site uses markdown for nearly all its formatting needs.
You can learn markdown in about 5 minutes. Here’s a cheatsheet to get you started!