How can I set up nested categories for a bookmarking application?

Hi

I'm trying to create a Bookmarking Application where information about each bookmark can be stored, i.e. Title, URL, Favicon, Screenshot, Description, Tags, and Category. And I'm having difficulties with the last of these.

I want the user to assign each bookmark to just one category which in turn may have one or more sub-categories, e.g.

Computing>Internet>Online Storage

On the main bookmark entry form, I only want the top level category field visible, with subsequent sub-category fields only appearing if applicable. I would also like the user to be able to add new categories at each level if required (after first being shown existing categories).

I'm not very technically minded and would appreciate any help anyone can offer on this.

Many thanks

Martin