When you post at your blog and forget to assign any category to it, wordpress itself assigns a category to it. This is called the “default post category”. However, you can change this default category. Also, you can change your default link category. In order to set your default post category please follow these steps,
1) Login into wordpresa admin panel.
2) Click on Settings -> Writing. [Check this image]
3) Select the category in “Default Post Category” field. [check this image]
4) Click on save changes to apply changes.