combo box: very urgent

Please someone help.

I have two combo boxes. The content of the second combo box should depend on the selection of the first combo box.

For example, combo box one has the following items: item1, item2, item3. If I select item1 my combobox2 will be populated with items that I want to related to item1 from first combo box and if the item 2 is selected another set of value is displayed for selection and so on.

This is very urgent and I would greatly appreciate any help.

Many thanks,
Debbie