After Adding a host check, you should configure the notifications for it:
On the left, choose Rules.
Choose the ADD RULE button.
Set the host, type of check, condition to be checked, and action to be taken.
For example, select a particular host, choose a Web site check, specify a condition that the check has failed 3 times consecutively, and set an alert to be generated.
Make sure the rule is set to Active and save it.
Choose Recipients on the left, and make sure you or someone else from your team is set to receive notifications for alerts and warnings.
If needed, choose Add recipient and specify the media (like e-mail or Slack) and address of a person who shold receive notifications for your hosts.
For more information about how to configure the notifications and the rules through which they are generated and sent, see Introduction to notifications.