how to get data on selection change of drop down list

You can bind the change event of your drop down list using a bit of jQuery like so...

var url = '@Url.Action("GetEmployeeData", "YourController")';
$("#YourElementId").change(function () {
    $.post(url, { employeeId: $(this).val() }, function (data) {
        if (data.length > 0)
            // Do something with the results , populate your textbox
    });
});

Whenever your list changes index, it will attempt to make an ajax .post to a controller method called GetEmployeeData.

You could then write a controller method like so...

public ActionResult GetEmployeeData(int employeeId)
{
    var employee = SomeObject.GetEmployee(employeeId);
    return Json(employee);
}

In the callback of the jquery script, read the JSON object and populate your text box