Making a Dynamic Form

Making a Dynamic Form


 Hello, I am very new to jquery, I'm more familiar with html and php; I've only used jquery before for menu animations, and now I need to make form with dynamic inputs, the second option on the form will depend on the input from the first option;
the best example is probably eBay or second hand car websites; an example of what I am trying to achieve can be found on the Autotrader website:  http://www4.autotrader.co.uk/
 As seen in my attached picture the "Buy a car" form features a 'make' field that determines the contents of the 'model' field.

 I've not been able to find any relevant tutorials or sample code online, can anyone lend a hand with getting me started? Just knowing how they may have made the dynamic 'model' part of the form would be incredibly useful.

 I'd need the second part of the form to retrieve its contents from a MySQL database depending on what the user selects on the first option...