Use "selrow" to get the selected row Id
var myGrid = $('#myGridId');
var selectedRowId = myGrid.jqGrid("getGridParam", 'selrow');
and then use getRowData to get the selected row at index selectedRowId.
var selectedRowData = myGrid.getRowData(selectedRowId);
If the multiselect is set to true on jqGrid, then use "selarrrow" to get list of selected rows:
var selectedRowIds = myGrid.jqGrid("getGridParam", 'selarrrow');
Use loop to iterate the list of selected rows:
var selectedRowData;
for(selectedRowIndex = 0; selectedRowIndex < selectedRowIds .length;
selectedRowIds ++) {
selectedRowData = myGrid.getRowData(selectedRowIds[selectedRowIndex]);
}