Help with If else Statement by using a drop down menu

Hi 

I am creating a fillable timesheet for work

I want to choose a option from a drop down boxwhere this will then give me the cost code for that option in the drop down box. I am new to Javascript.

for example if dropdown1 = supervision then textbox1 = 1234456

I have a couple of items in the drop down box for example

Supervision      =        1234456

Sick Leave       =         789654

Annual Leave  =          456123 etc.

thanks in advance