First, you need to log into your account. This can be done in the hamburger menu.
At the bottom of the app there is another menu. The second icon on the menu looks like a bulleted list. If you click that you get a search bar where you can search for communities to subscribe to. I used this site to get a list of popular communities: https://browse.feddit.de/
Then, in the homepage, you can use the top menu to view all subscribed community.