Mastodon does not have an algorithm to recommend content. Instead it provides you with a feature to "follow" hashtags, and will populate your feed with any posts that have these hashtags.

Begin by searching for hashtags on topics you are interested in, e.g. #fediverse. In the search results it will list matching hashtags and matching accounts.

1. If there are any matching hashtags, click on the hashtag search result,
2. then at the top of the next screen you should see a button that says "Follow this hashtag"

Repeat these steps for every hashtag you think you might like to see.

The advantage of following hashtags is that you will see all posts from all people that post with that hashtag, even if you are not following those people. This allows you to discover people who regularly post content that is interesting to you.