In order to send the newsletter via an email campaign platform, you must have the Email View Add on enabled. If you do not have this Add on, you can send a link to the newsletter URL via your platform of choice.
Sending with Campaign Monitor
If you have the Campaign Monitor Add on enabled, you can send the newsletter from within the Dashboard. Read the guide to learn how to send your campaign with Campaign Monitor.
Sending with Mailchimp
If you have the Mailchimp Add on enabled, you can send the newsletter from within the Dashboard. Read the guide to learn how to send your campaign with Mailchimp.
Sending with another platform
If you do not use any of the options above, chances are you can still send your newsletter with your current system if it supports importing HTML via a URL. Check with your provider to see if they have this option.
The Email View Add on page has further detail on how you might import your newsletter to your system for sending.