Just a quick guide to something that should be easy but can be a pain in the A.
First go to Appearance>Themes
Click Add New. You will see a list of some popular free themes that you can try out. You can even click Preview and see how it would look on your site (though normally it will look shit until you mess about with it.)
But we want to add our beautiful premium themes. When you buy one, you get a zip file.
Click Upload Theme.
You'll see this:
You can drag the zip folder onto the 'choose file' button and that should work. Or you can click on it and use the file manager to find the file.
That's basically it.
With Genesis you will do this twice - once for the Genesis Framework, and once for the theme.
Here on Britain's Smallest Shop you can see I've got two themes installed:
Smart Passive Income Pro is the active theme and Genesis is there too. If I deleted Genesis it would break the site because although it doesn't SAY it's active, it is.
Note that I deleted every other theme and there are only 2 there. I don't want those other files clogging up my sites and slowing down the backups etc, so I keep things lean.
Note also that I don't need to take further steps in terms of creating a child theme - the Smart Passive Income Pro theme is acting as the child theme, as will any other Genesis theme.
The theme you add must be a Genesis theme. Basically one from this list.