How much experience do you have creating scripts in FileMaker. I suspect that you have limited experience in that. If so, you may want to study up on scripting via a training tutorial or book on the subject. We'll be glad to help, but the added training will help you fill in background info and get you up and running much quicker.
Selecting data from a drop down will put that data in a field. Your script can then refer to that field in order to use the data selected in that field. There are many, many steps that do this in different ways so it's hard to be specific. If can check the value of a field. Set Field, can change it. Just to name two examples.