As the admin user of your Mastodon instance you enjoy extraordinary powers. To ensure these powers are safeguarded we highly recommend you enable two-factor authentication.
Go to Preferences > Account > Two-factor Auth to enable two-factor authentication.
You probably want to familiarize yourself with Mastodon before you start accepting new users. With this in mind we recommend you temporarily disable new user registrations.
Go to Preferences > Administration > Server Settings > Registrations and set Who can sign-up to Nobody can sign up for now. You can re-enable it later.