Start by searching/following hashtags. Find accounts through boosts etc. There are also accounts focused on boosting interesting accounts. Once you have a bit of a network established, more accounts come into view through those you follow.

It's actually a great thing about Mastodon. Just come in and start following whoever. Never have a feed full of "top posts" or whatever garbage. I could not care less about what some algorithm thinks is trending... If the people I'm following are interested in it, it will make it to me anyway.

Engage with topics you're interested in and follow people who seem cool. Following hashtags has worked well for me.

There are some guides online. If you want to know more: https://fedi.tips/how-do-i-find-accounts-to-follow-on-mastodon-and-the-fediverse-how-do-i-find-my-friends/