To create a new post, all you need to do is create a file in the _posts directory. How you name files in this folder is important. Jekyll requires blog post files to be named according to the following format:

YEAR-MONTH-DAY-title.MARKUP
Where YEAR is a four-digit number, MONTH and DAY are both two-digit numbers, and MARKUP is the file extension representing the format used in the file. For example, the following are examples of valid post filenames:

2011-12-31-new-years-eve-is-awesome.md
2011-12-31-new-years-eve-is-awesome.html
2012-09-12-how-to-write-a-blog.textile