Drop Down To Pull Record From Database

I am trying to create a form to edit existing data in my database. I have 2 drop down boxes( category then item) they do not work correctly. I am using php to populate the drop down boxes but there is nothing dynamic about doing this. I think I need to totally rewrite the page using java script or Ajax but I don't know what parts.So I want the user to select a category from a drop down list populated by database then have it populate the item list(so I think both of these need to be java script in case they change the category it will re populate the item list). Then I just want the selection of the item to pull the information to my form so they can update it and save the update(which I am pretty sure I can save the update as a php function but need to populate the item editable information in java script). I am really looking for help on how to setup the page with java script to make database queries and use the results to update parts of the page.In case you confused here is my flow:page loads

-User selects catagory query database for item list (on change event it think)
-User selects item.Item information is pulled and displayed from database(cost, serial number, quantity(another on change event)
-User is able to edit information and save updated results(run an update.php...)