tdo forms – Selecting parent category then child category

I would like to be able to have a user select the parent category (state (US)), then the category dropdown (city – child of state) would only give the city options for that particular state, and then from that city select from the child categories for that city so users don’t have to weed through hundreds of categories. I know this is probably easy if you know your way around the code but I do not – I do know a little, but obviously not enough yet. Any help would be greatly apprectiated. Thank you.