I am working on a nopCommerce project now. I think it'd be good for me to write down the steps for doing certain things, so it will help me to remember how to do these in future, and also hopefully it might help others too.
1. Click on the Administration link in the home page.
2. Select Catalog > Categories > List from the menu.
3. Click on Add New button to add new category.
* You can define the sub category by selecting its parent category.
* You can choose whether to allow customers select the page size. If not, then you can specify the page size.
* If you want this category to be displayed on the top menu, check the Include in top menu option.
4. Click on Save button.
You'll see the list here:
When you go to the home page of your store, you'll see the categories listed in the top menu (if you select it to be).
** Please note that I am just a beginner in developing nopCommerce e-commerce website. My ways of doing might not be the perfect or very good method, and just how I am working on them currently while learning.