categories and subcategories dropdown

Question

Hi everyone! I am very fresher to asp.net. I want to build web sample which write with asp.net c#. In my webpage it's include three drop down:

first drop down is location:(example, city or town)
second drop down is Main Categories:(example, cars ,clothing, cosmetics,etc..)
third drop down is Sub Categories:(example If I choose cars in Main Categories then it will display brand new cars,used cars which are within in chose location area).


Really thanks for your precious time and helps!have a nice day!