jQuery Grid Plugin - jqGrid

Hi,

I am a newcomer to jqGrid and I am stuck up with sorting in my grid.

Below is my code for jqGrid.The data is displayed properly but still i am not able to sort the columns.

$(function () {
            $("#usrsearchgrid").jqGrid({
                datatype: function (pdata) { getData(pdata); },
                height: 250,
                viewrecords: true,
                pager: jQuery('#pager'),
                loadtext: 'Loading data...',
                recordtext: "{0} - {1} of {2} records",
                emptyrecords: 'No User exists',
                pgtext: 'Page {0} of {1}', //Paging input control text format.
                colNames: ['country', 'emailid', 'firstName', 'lastname'],
                colModel: [
             { name: 'country', width: 150, index: 'country', sortable: true },
             { name: 'emailID', width: 200, index: 'emailID', key: true, sortable: true,sorttype:"string"},
             { name: 'firstName', width: 200, index: 'firstName', sortable: true },
                { name: 'lastName', width: 200, index: 'lastName', sortable: false }
            ],
                sortname: 'emailID',
                sortorder: "desc",
                rowNum: "2", // PageSize.
                caption: "JSON Example"
            });
        });
        function getData(pData) {

            $.ajax({
                type: 'POST',
                contentType: "application/json; charset=utf-8",
                url: 'JQGrid.aspx/getUsers',
                data: '{}',
                dataType: "json",
                success: function (data, textStatus) {
                    if (textStatus == "success")
                        ReceivedClientData(JSON.parse(getMain(data)));
                },
                error: function (data, textStatus) {
                    alert('An error has occured retrieving data!');
                }
            });
        }
        function ReceivedClientData(data) {

            var thegrid = $("#usrsearchgrid");
            thegrid.clearGridData();
            for (var i = 0; i < data.records; i++)
            //grid.jqGrid('addRowData', myData.length + i, newData[newData.length - i - 1], "first");

                thegrid.addRowData(i + 1, data.rows[i]);
        }
        function getMain(dObj) {
            if (dObj.hasOwnProperty('d'))
                return dObj.d;
            else
                return dObj;
        }

Can you please suggest what additions should i make in this..??