Just create a new file in the _posts directory with a filename of the scheme YYYY-MM-DD-title.md (file ending .md is for Markdown but other filetypes are possible) and add a YAML front matter, which is two lines of three dashes each, with some information about your post in between:

---
title: "My newest post"
tags:
 - soup
 - conferences

---

This is about soup, and conferences.

You do not necessarily need to enter any information. You could leave just an empty line between the dashes. But a title and tags make the log functional.
The date of the post is extracted from the file name, but you can overwrite it by specifying a date in the YAML front matter, e.g. If you require a higher resolution because you do multiple posts a day.

Also, you can add the parameter updated: which adds this information to the post. For this page the front matter is:

---
title: "Creating a New Post"
date: "2014-10-07"
updated: "2014-10-08"

---

Posts are typically written in Markdown (Jekyll by default is using kramdown as a parser, but that can be configured).