On Mastodon, you can report a rule-breaking post by clicking … on the post and selecting Report.
Alternatively, you can report an account by going to the profile and click the ︙ or … button and select Report. However, if you do this it’s important to include examples of what the account has done wrong. Simply reporting the account with no examples creates a lot of work for the moderators.
Mastodon reporting forms include the option to also send an anonymous report to the server of the account that wrote the post. This can be really important, because only a user’s home server has the power to suspend or delete their account. Other servers can block accounts, but in the worst cases it may be better that a nasty account is deleted at source.
Reporting anti-social accounts is a good idea as it’s the main way server administrators find out about nasty behavour. Once admins are made aware of a problem, they can take action using special blocking tools that are not available to ordinary users.