I recoment that you read my Roling with Servage before going on with this guide.

Sources:
http://wiki.rubyonrails.org/rails/pages/HowToSendEmailsWithActionMailer
http://email.about.com/od/emailprogrammingtips/qt/et_mail_rails.htm
http://www.stephenchu.com/2006/06/how-to-use-gmail-smtp-server-to-send.html

Guide to copy:
http://wiki.rubyonrails.org/rails/pages/HowToSendEmailsWithActionMailer

But please note, what servage, uses sendmail, so the configuration described in
"Configuration"
Should be replaced by this:
File: /config/environment.rb
Incert at the bottum:
ActionMailer::Base.delivery_method = :sendmail
ActionMailer::Base.raise_delivery_errors = true

File: /config/environments/production.rb | development.rb
Bottum of file should be:
# Disable delivery errors, bad email addresses will be ignored
# config.action_mailer.raise_delivery_errors = false
# config.frameworks -= [ :action_web_service ]
config.action_controller.session_store = :active_record_store

When you are set to go.

You can combine the action with when people signup.