Start by “following” hashtags. Type in hashtags for things you like, e.g. “#mastodon”, and search for it. When the hashtag shows up in the search results click on the follow button to the right of the result item.

It is a good way to receive content from people you are not following. If you see people posting to that hash tag often, check out their profiles to see if they have a good post history, and then follow them if you want to see more. If you follow more people, and you also have good posts yourself, more people will follow you back.