I already replied with this text in this thread, sorry if you guys think its spam but to make sure you also read it:
Steemit supports HTML code in your post. So basically u can use any code from here. In my opinion its much better to use this instead of markdown or something like a combination from both of them because it is so easy to look HTML code up. Alternatively, this post is just some tips from the Markdown Styling Guide which is also linked above the preview window.
Hope this was useful for you. If something is not clear please make sure u ask me no worries :)