Looking for help in coding arrays and dropdowns

I am currently trying to create about 3 dropdown menus that display specific things in them based on previous answers. It is mainly to do with factions, so i have a list of factions and within those factions are characters, troops and machines. So i want the second dropdown to have these three categories, and then based on the faction selected and the category selected it will display a list of units. I've tried working with it for a while now and looked at arrays, but i don't have much knowledge within this subject area so i am quite out of my comfort zone and don't really know what i am looking for. I just know the concept as explained before, if anyone can help me out it would be much appreciated. Thanks.