After the registration process, it's time to create your first channel. You will have to choose if you want it to be public by default ("Public") or followers-only by default ("Personal"). Don't worry, this setting can be overridden for individual posts if you wish.
Find your first contacts. Since you are here, you probably already know some Hubzillians. (If your server supports Activitypub you can also add contacts from Mastodon, Calckey, Friendica).
Those are the necessary steps, simplified. Hubzilla is a huge toolbox with a lot of possibilities to fine tune your settings. But for your first try - this will be enough.