How to add items from dropdownlist to a specific category?

Hi,
I'm trying to find out how I can add selected items from dropdownlist to to a specific category. See below for pictures.

1. This is where I add Info about thing. (Add.aspx)
 

2. Here, As you see I want to add selected item from dropdownlist to specific category to the left. Lets say I choose "Book" from dropdownlist and I want them to be placed on Book
category, and Superman to be placed on Superman and so on.. (Main.aspx)


Does someone know how i can implement this?

- Per