An incomplete thread of cool things I'd want to know about if I had just joined #Mastodon:
✨ People are pretty great here! In my experience, you can just click "follow" on any random person who posts one (1) thing you're interested in, and 99.8% of the time you'll be glad to have their posts/boosts on your timeline. You can always unfollow them later.
✨ You can follow hashtags too! Click on the tag, then the little plus sign at the top. New posts from the tag will show up in your Home feed.