The title has two formats:  
First: Use `=` or `-` ,with at least one symbol. 

I am a first-level heading.
=
I am a second-level haeding.
-  
Second :Use `#`  
One `#` represents first-level title,two represent second-level,and so on.**No more than six levels in total.**
# I am a first-level heading. 
## I am a second-level heading.