Need script to populate a text field based on the user's choice from a drop-down menu.

I have a scripting question.  I don't this will be difficult but I can't seem to get it to work.

Here it is:

Question 1 (drop-down menu with 2 choices -yes or no).

If the user chooses "no" from the drop down, I want the next question to automatically be populated with "N/A".

What is the script for this?  If statement? Also, what script event do I place it under?

Thanks a bunch!!!!!