DropDown With Onchange Event

I want to ask about if i want make dropdown with onchange event, so basically for every diferent option in dropdown value it will show diferent data, anyone can help me with the basic code?

Hi here is a simple implementation in jQuery

PHP Code:

<script>
 $("#dropdownUL").change(function () {

var valueSelected = $("#dropdownUL :selected").val();

// Do something

switch (valueSelected) {
    case "new":
        $('#newDiv').addClass('show');
        break;
    case 'new2':
         $('#newDiv2').addClass('show');
        break;
}


});
</script>