Filter by category then sub category

God help me please!

[whining]
I have been trying to figure this out and I understand it is probably something easy that no one even waste their time solving!

[problem]
How can I sort posts by category and then sort that category by a sub category all on the same page?

[In-depth]
I want to be able to select a parent category to display all posts of that category on a page. Then I want to have a drop down menu that shows the sub category of that category page and lets the user on the front end select it and filters out all the other selections. After selecting the first subcategory a second dropdown should appear to allow for further sub-subcategory filtering and so on and so forth.

[TL;DR}
I want a frontend tiered/hierarchical post filtering system that I can put on a page.

Thanks