Go to admin/config/system/actions and Create an advanced action, selecting Send an e-mail.
At the bottom are all the tokens available to insert in your text.
(Note: Not all of these tokens are working / maybe incorrect, maybe buggy: http://drupal.org/node/870868
Label:
Send e-mail to administrator for new user
E-mail:
admin@example.com
Subject:
New user has signed up [user:mail] [author:mail] [user:name]
Message:
[user:name] created.
Mail: [user:mail]
Then go to Triggers.
admin/structure/trigger/user
Note Actions are stored by number rather than machine name (unfortunately). When you add an action it is given a crazy temporary ID that you can see in the URL, such as admin/config/system/actions/configure/GPp2_l4kooAxc5SprQBkGqXtyDyNLa59BLwSWoqnAqE, and when configuring, you will be at a URL similar to admin/config/system/actions/configure/26