Selecting a Root Category:

Navigate to Settings > Catalog > Active Catalog 

You can set a category as the ROOT category, but you can only have one per store.  To do this, click the "Is Root" checkbox, then click save.
If you select "Home > group test >" in the screenshot below, you would only see everything beneath Home > group test  in your store.  The user never sees the root Home > group test category, only its subcategories. 

Hiding a Product Category:

If you are running multiple storefronts, you may want to show categories on a particular store.  There are a number of ways to make this happen.  You can set a root category on a store.  This says that anything from that category down is the primary category group for that store.  The other option is to show/select the specific categories that need to be shown on a store.

1. Go to the "Settings > Catalog > Active Catalog

2a. Select the store you need to work with. 

2b. Uncheck the "Show All Catalog" check box.

2c. Click search to list Categories

4. Check the boxes to the left of the categories that you would like to display.
5. Click on the "Add Selected" button.
6. Click the Save button located beside the "Show All Catalog" check box.

Your site should now display only the categories that you have selected.

Hiding the word "Root"

You can hide the heading in this category, in the category layout you can select in live Design > Theme Edit > This Page (Category) HTML Editor.

You will need to take out the following: <div></div>

You can replace it with <h1>Your Text</h1>

Also, you can change the name of the root category by creating a new category for all the sub categories to go inside of. So those are the two options you can use to achieve this functionality.