An Email Template is a container for all of your Emails. You will need to create one before you send out your first Emails.
To do this, in your Email Square screen, go to Templates > Add new Template.
The editor has many options, including writing sourcecode. Also, the placeholders are a very good way to make your Email personal.
When you are finished, we recommend that you test your Template before using it. You can do this by pressing "Send Test".
When you are happy with the design, press "Save".