You'll use those categories to setup the links your users can use to find your pages, as well as the way they're indexed by search engines. Don't forget to tag your posts as well.
WordPress Design Themes - Now, do you want the generic WordPress Theme that comes with WordPress, or a custom designed one that is specific only to you? The default WordPress theme is perfect if you're just setting up a blog to have a blog. BUT, since you're in this industry to make money and brand you, I suggest a designed custom theme. There are tons of people who design custom WordPress Themes that you can easily upload to your domain to change the Generatepress appearance and feel of your WordPress website. Some of the WP Themes are paid versions, but most of them are free.
A paid theme generally comes packed with the features you need, and you just need to pick and choose which ones you want to use. You can then customize them to your needs in a matter of minutes.
Connect to your web server via FTP. I use the free Filezilla client, but any FTP software will do the trick. You will need to obtain the FTP hostname, username and password from your web hosting provider if you don't already have it handy. You can usually find it by logging into your web hosting control panel. Contact your hosting provider if you get stuck on this.
So, why would anyone ever pay for a theme when there are so many free themes available? A big reason is that you may find yourself wanting to customize your site down the road and it can be time-consuming and frustrating to try and "hack" your theme in order to do so. The alternative is to purchase a premium theme, which is usually priced in the $30-100 range for a single-use license. Premium themes often include lots of options making it easy to customize, plus available support and documentation if you end up with questions or issues.
There is also another option that will tell you how to install a WordPress theme. Use the built-in theme uploader, which can be accessed directly from website the WordPress Administration Panel. Go to the appearance section in WordPress and choose the 'theme' section. Once there, you can click on 'add new them' to get started.