In order to start sending emails, you must activate the campaign. You can do that anywhere you see the Activate button.
Once the campaign is activated, you will see some statistics appear on the Summary panel.
And if everything is ready to go, you should see this status message
Note that you might see other messages instead. For example, if the Send Window is closed, you'll see this message:
You can fix that by adjusting the Schedule.
You can see if your campaign is sending by going to the Steps panel and looking at the statistics
If you have any manual steps, you'll also need to click Preview and Send to kick off emails to your prospects.