When you're starting out with a totally empty timeline, the directories are probably best:
https://fedi.directory
https://communitywiki.org/trunk
https://fediverse.info/explore/people
Once you've got some people in your timeline, you'll see posts shared and people mentioned in discussions.
There's the Explore tab (on the website) or the magnifying glass ? icon on the official app.
Also, when it's relesed, following hashtags will help a lot.