Administrator notification on user registration
First, go to /admin/config/system/actions. In the section "Create an advanced action", choose "send an email" and click on "create". It takes you to a page where you need to enter a recipients email address, (enter the Administrator's email address here), subject and message. Click Save.

Second, go to /admin/modules and enable the Trigger module.

Third, go to /admin/structure/trigger/user. In the Section "Trigger: After creating a new user account" Choose "Send e-mail" in the select box. Click Assign. 

Example Message:

A user has created an account.

Details:
 - Username: [user:name]
 - Email: [user:mail]
 - Time: [user:created:long]

Activate their account by going here: [user:edit-url]