An automation will set off whenever your contacts trigger it. Say you've created an email follow-up series - any contact subscribing to your list using an opt-in form will trigger that sequence.
However, if you already imported contacts to your list and you'd like them to trigger your automation as well, you can enable it manually for those contacts only:
1) Go to the "Contacts" menu, head to the list your automation has been associated with and click "Contacts". If you don't find your list there, click "View all" (on the left-sided menu).
2) A page will come up with all contacts in that list. Simply tick the checkbox to the left of each contact you want to add to your automation and hit "Actions" in the lower left corner of the page (if you'd rather have a whole segment triggering your automation, just load that segment and hit "Actions for this segment").
3) Click "Enable automation", select your automation and save your changes.
And that's basically it! All actions you set for that automation will now run for the contacts you've chosen here.