firstly welcome jass.
wow, you have not given us much to go on, have you?
the purpose of a listbox is to provide a list of acceptable responses in connection with a field in a table. so lets say we have a table of user names. one of the fields is Title, the options might be Mr, Mrs, Miss, Sir, Dr .... So on a form used to enter a new user you would have a listbox with your choices and set the listboxes Control source property to the name of the field - Title.
if you could provide more detail (what values are in the listbox, what do they represent and where would you store that value) i am sure someone will help you more.