In the Web Portal, go to the Settings Cog > Integrate > Mailchimp.
(Note: If you don't have a current integration with Mailchimp then you will be prompted to create one.)
In these settings, you can decide which contacts to have added to the Mailchimp Audience by selecting from the listed tags for the Contact Types to Sync.
The email information for contacts who have the selected contact type tags, will automatically integrate with Mailchimp and become a part of the Mailchimp Audience.
You can then scroll down to Sync Contacts to Mailchimp Audience. The name of your Mailchimp Audience should appear in the dropdown. You can select that, and emails will be sent to every contact in that Audience list in Mailchimp.
This will set up the Audience that your emails will be sent to.