This is done by creating nested sub-forums (e.g. create a "Computer" sub-forum and then further sub-forums within that for "Software" and "Hardware".)
More importantly, for someone intending to build a structure like yours, Nabble offers a number of ways for presenting a forum, which it calls "Forum", "Board", "Mixed", "Category", etc. It is probably best not to try these alternative "applications" until you have your basic structure in place, since not all the changes in appearance are obvious until you have the sub-forum names, descriptions and one or two posts in place.
As you play with the various applications you'll see some of the terminology will change. For example, what is described as a "Sub-forum" in some places becomes a "subcategory" in others, but essentially its referring to the same thing.
To create a sub-forum be sure you are logged in under your Administrator username (and not any additional user you may have created for testing purposes) and go to OPTIONS > STRUCTURE > CREATE NEW SUB-FORUM
To change the general presentation go to OPTIONS > APPLICATION > CHANGE APPLICATION TYPE.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.