This tutorial assumes you've already logged in to WordPress, and will show you how to manage your categories.
Click Posts in the left margin.
Then click Categories.
This is where you can add a new category... let's do that now.
When ready, click Add New Category.
You can quickly edit a category's information by hovering over the category name...
Then clicking Quick Edit.
You can change the name and/or slug of the category from here.
You can also delete a category this way.
This is the end of the tutorial. You now know how to manage categories in WordPress.