Related selectbox problem

Dear All,

I wanna make 2 selectbox which is contain CITY & Location. I have 2 table in my SQL :

     1 - CITIES 

          ID = city code

          City_name = Name of city

     2- Locations

          ID = Location code

          location_City =  CITIES Table / ID Field

          Location_name = Name of location on selected city

now I need to have 2 select box which the first is CITY name and second one contain Locations

     when i select City from CITY select box on second box just name of related location of selected city coming.

Please help me how can i do it

Thanks