Javascript on form

Just wondering if someone would be able to help me with writing a java Script for a form. I basically want to do a If ... then... statement. Apparently business rules don't work on the web front end.

Basically, we have a form on the web - Depending on what values a person picks on two fields on the form, i want a message to be displayed. IF they do not select these two values, then i don't want anything to happen.

So the two fields are nationality & product. So, for example if the person picks product A and their nationality is Irish, then i want a message to be displayed, otherwise i don't want anything to happen.  Any help appreciated!