There are lots of not so easy to understand markdown guides, but mine is one that's pretty straightforward and makes beginning using markdown super easy. With all the new steemit users coming onboard I want to make their first post super easy.
Markdown code: Characters to use before and after a word or words to affect how they look
Headings
# is equal to 160% text size on on steemit
## is equal to 140% text size on steemit
### is equal to 120% text size on steemit
#### and ##### are recognized but will give you approx 100% text size
Surround you word or words with these characters for effect:
* for italics *
** for bold **
*** for bold and italic ***
Insert images, links, and youtube videos by just copying and pasting the link
http://www.somecat.xyz/cat.jpg gives us 
https://steemit.com/ will result in https://steemit.com/
https://www.youtube.com/watch?v=aKTH0o8AEtQ embeds the youtube video
Quoting can be done with a
>at the beginning of a line
Headers can have bold and/or italics effects
use ### *** to begin and *** to end the line
___ or *** or --- gives up a horizontal rule