Codeigniter with Ajax

[eluser]Joshi[/eluser]
This is a very simple javascript. I have a drop down and a two text boxes. I am designing a site related to payment. The drop down will have a list of account number for a user. When the user selects the account number from the drop down, i need to fetch the details say Bank Name, Routing number for that account number and fill in the text box with these values. The text boxes should automatically populate with the values based on the drop down select. Can u provide any sample link or code to do this?