I have a form which contains a select named Client_ID as well as some other text inputs. What I need is that when a user selects a Client_ID, my fields Address1, Address 2 etc s
Read this tutorial. I think it might help you understand the concept of moving data from client to the server and vise the versa.
Keep in mind that in your case you need to combine event.
$(document).ready(function(){
$("#my_select_box").change(function()
{
//send the data to the server as in the tutorial above
});
});