Writing blogpost:

Create a new markdown file in _posts folder and name it following this format:
YEAR-MONTH-DAY-title.markdown

Before you start writing you need to add:

---
layout: post  
title:  "Tested"  
date:   2016-11-23  
categories: test  
---  

in the beginning of your markdown file.
Where
layout is the html file you want to use in the _layouts folder.
title is the title of your post.
date is current date.
categories is to which categories this post belongs to.

Adding category:

If you want to create a new category which doesn't exist yet, you need to create a html-file in the blog/categories folder, ie categoryname.html

example file test.html for category test:

---
group: blog
layout: category
title: test
description: All blogposts published under test
permalink: /blog/test/
category: test
---

{'%' include categories.html '%'}

layout is the html file you want to use in the _layouts folder.
title is the title of your category.
description is the description of your category.
permalink is the link to the category in the browser.
category is the current category when fetching posts.
{'%' include categories.html '%'} includes the file which will show all posts in this category. Remove the '' from the % signs.