Hide Part Of Page

I have created a drop down box that picks items (active product lines) from my dbase (sql), once the user selects an item from the drop down, I would like asp to relate that choice to the next table and pop up another drop down box (active products from the above product line chosen).

I'm assuming that there needs to be some function to hide the second drop down box until the first choice is made (I could be wrong here??) so I don't know what the code would be for the page to stop showing anymore information until a choice is made. I usually show code here but this is really a question about code that I don't know.