Newbie to ajax Need help

Newbie to ajax Need help

I am learning by creating a sample application. As part of it I have used a sample of php pdo crud which I adapted to my sample. I want a code for the following. when I click create in the crud opens a modal form. All these are ok. What I need is the code where In the form, I need to accept a value using a bootstrap select. For example when I select north south east and west in a select it should display the branch address in anther select form element from a mysql table which has branches in north south east and west. I request the code part as I am unaware of ajax. And how to incorporate. Thanks