Click the community name above the post, for example [!liftoff@lemmy.world](/c/liftoff@lemmy.world). You should see a subscribe button there.

You can search for a community by going to the app's main screen and pressing on the magnifying glass icon. There, select your instance and enter a search query. From there, select Communities for a list of communities from your search result. If you want to discover communities, just enter a space in the search query and hit search, it should give you a list of all communities (in that instance I suppose).