AJAX/PHP/MySQL - Simple Database Script

I have just created a WordPress site and added two new tables to the database. On the home page of the site I need two sections, where visitors choose two things to compare. Each of these two sections will be the same: - One drop down box (like a category - all of these categories are in a new DB table) - A second drop down box that, after selecting a category from the previous drop down, appears containing the items listed in that category (all items are in a new DB table) Once all selections have been made, the visitor then clicks 'Compare' button and the results will be shown below the drop down sections. All of this will need to be done WITHOUT the page reloading at any point. This project does NOT require you to do ANY design work, all I need is you to get the drop down boxes and their values, make the secondary drop downs only appear when a value has been chosen in the first, and then return all DB values for the chosen items when the button is clicked. Cheapest bid that I believe can do what I need will be chosen.