ERROR: Google Maps API MissingKeyMapError in ASP.Net

I tried to implement the google map auto complete but its not working

my code is

<asp:Content ID="Content2" ContentPlaceHolderID="contentMain" Runat="Server">
		    <span>Location:</span>
    <input type="text" id="txtPlaces" style="width: 250px" placeholder="Enter a location" />
    <script type="text/javascript" src="http://maps.googleapis.com/maps/api/js?sensor=false&libraries=places"></script>
    <script type="text/javascript">
        google.maps.event.addDomListener(window, 'load', function ()
        {
            var places = new google.maps.places.Autocomplete(document.getElementById('txtPlaces'));
            google.maps.event.addListener(places, 'place_changed', function ()
            {
                var place = places.getPlace();
                var address = place.formatted_address;
                var latitude = place.geometry.location.lat();
                var longitude = place.geometry.location.lng();
                var mesg = "Address: " + address;
                mesg += "\nLatitude: " + latitude;
                mesg += "\nLongitude: " + longitude;
                alert(mesg);
            });
        });
    </script>
        </asp:Content>

Please help me out???