What you describe is called a conditional value list.
For starters, let me know if you've already read any of these sources on conditonal value lists:
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
If that doesn't get things working for you, please post back and tell me where you've hit a snag in figuring this out for your database.