jQuery to Change Form Elements

jQuery to Change Form Elements

Am looking to design a form that deals with items in specific category. The form will have one dropdown box that dynamically displays data in a DB. Ok, check thats done. My problem is; when a category is selected, i want another dropdown box to appear on the page without requiring a refresh. This second box will display sub categories that also are stored in the DB. I have been told that jQuery is the best to use for this, so i have been really getting into it lately but cannot manage to generate the code to make this work. Any ideas, example pages, or code would be great!

Thanks!