Help creating action
You don't need to create this action programmatically. You can simply use the Rules UI.  I'm pasting an export of a sample Rule that does this. You can import it using the Rules UI import link.

{ "rules_email_administrators_when_new_node" : {
    "LABEL" : "Email administrators when new node",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "ON" : { "node_insert" : [], "node_delete" : [] },
    "DO" : [
      { "mail_to_users_of_role" : {
          "roles" : { "value" : { "3" : "3" } },
          "subject" : "Node added or modified.",
          "message" : "Hi, administrator(s),\r\n\r\nA node has been added or deleted.\r\n\r\nNode ID: [node:nid]\r\nNode title: [node:title]\r\n\r\n"
        }
      }
    ]
  }
}


EDIT: Now, if you MUST do this programmatically, I would first look at the Examples for Developers project (it contains an action example). In a nutshell:


First, I would use dpm($context); and 'dpm($object); to see what parameters are being passed to the function. You can probably use some of the information there to include in the email, like the node ID and the author ID.
Then I would load an array of administrator users, maybe using EntityFieldQuery or a db_query. 
Then I would use drupal_mail() to send the email. It shouldn't be hard to do.