Here in the Las Vegas area the local water company is the Las Vegas Valley Water District.

Their office is located at:
1001 South Valley View Blvd.
Las Vegas, Nevada 89153

Customer Service Phone Number:
702-870-2011 or (800-252-2011

Business hours:
8:00am to 5:00pm Monday - Friday

Information on turning on service can be found online at the following link:
http://www.lvvwd.com/html/cust_serv_turn_on.html

The following link will take you to the Las Vegas Valley Water District's Web Site.
Las Vegas Valley Water District