How to create a Dropdown field based on information on another field.

Hi guys.

I have a question that im hoping someone here can help me with.

I’d like to link 2 drop down fields together.
What I mean by this is, Eg field one has countries. and field 2 has states. I want to add all countries and states to a category and have all the states in that country as a sub category.

Basically, When you select a country on the 1st drop down. It will only display the states in that country on the 2nd drop down.

I know this is probably confusing. But basically I’d like to know what type of code I’d need to pull info from a Category or Subcategory.

Knowing this would put me in the right direction on resolving my issue.

Thanks