This one is very easy. Just add a call to the mailer once a user is saved in the create action of your registrations controller (assuming that you're extending from Devise::RegistrationsController).
Follow this for a more thorough guide: http://guides.rubyonrails.org/action_mailer_basics.html
Note - you'll need to have an smtp server, and you'll need to set the credentials for that server in your config. So either build one yourself (bad idea), set it up with your gmail account, or use a Heroku add-on like MailTrap/MailGun.