So you're just getting started in Mastodon huh?

Here's a neat thing. On the web interface you can search for a Hashtag.

Same as you can on Twitter.

Something like #Ukraine, for example

But on the search results, see that little user icon on the right? Click it and you'll follow it as if it's a user!

New posts with that tag will arrive in your home page, even if you don't follow the author yet

@Pwnallthethings does it matter what server you follow - re: what you see, or no?
@Pwnallthethings @awiley you have three main timelines. Your home timeline is the people and hashtags you follow, your local timeline is everyone on your server whether you follow them or not, and the federated as I understand it is All the public posts from across mastodon from users that are followed by people on your server. So two of your three main timelines are influenced by the server that you joined.