I hope everyone is prepared for the fact that this place will soon be overrun with assholes once they figure out it's where people are going. It always happens and it will be a very serious challenge for the moderators. Gird yourself.
On Mastodon, we are all moderators in a way. Click the 3 dots below the toot and you'll see the options to mute, block and report the user. I think if you click report yo see an option to report to your instance admin or to the admin of their instance. But I don't want to try it because I haven't run against anyone I want to report yet.
There's also an option to block the entire instance if you find it's full of assholes.