Before you start emailing on NationBuilder, it's important to have a custom domain and email addresses that match that custom domain. We've made this easy for you with a guide to setting up a custom domain.
We also have some ideas for building up a good opt-in email list for your effort if you don't already have one:
You can import existing email contact lists by following the steps here:
After you have your custom domain and email address and an address list, you can set up your first email under Communications by selecting the representative of your nation you want the email to come from and clicking on "New blast."
There are a bunch of resources on your Communications options here.
Also see this guide on emailing with NationBuilder - nationbuilder.com/how_to_setup_email