One way is to just follow hashtags and see whoposts to them. Since hashtags are basically the de facto way to find relevant content on Mastodon, not just a marketing tool like on Twitter. And then once you see who posts interesting stuff, you can then add them to your follows.
Another way is to check out instances that talk about things you’re interested in. There are several websites that list them but here is a shorter list: fedi.gardenCheck out their feeds and see who posts things you’re interested in.
Furthermore, you can follow a.gup.pe accounts, called “groups”, which work similarly to hashtags. Each a.gup.pe account is basically a repeater that boosts (i.e. “retweets”) every post that pings it, so that anyone following it gets that post. For example, I follow @climate, and every time someone pings that (it’s like including a hashtag), it’ll boost that post and I’ll see it too.