how to show the message using java script when i click the combo id

i send the atttach file of run mode.

in that when user clicks the Net  Payable Days Combo box first  i want to show the message using javascript as please select the employeeId first and then Select the Working Days.


Net Payable Days textbox combobox

please send the code.it will be useful for me.i want the code using javascript.

Regards,
RAO