If the Host of your Mighty Network has enabled Topics, you will be prompted to follow some Topics when you first join. When you follow a Topic, your main feed will be personalized so that you see more of the posts in that Topic. You'll also automatically be introduced to other members who are interested in the same Topics.
If you ever want to update or change which Topics you follow, just navigate to Your Profile and tap on the Edit link in the upper right corner
Then, scroll down until you see the section that says Followed Topics. Tap on the pencil icon at the bottom of the list of Topics to follow or unfollow any of the Topics available in the network.
When you're done, return to the list of Topics in your profile and tap on Save in the upper right corner to save your changes.