Populating a drop down list using filtered data from DB

As of this point all I know is to bind the Drop Down List to the database. Now, I need to show information pertinent to a client. My code is pasted below and as you can see I did not get very far with it. I do not know how to connect to the xsd object.

 

The project is setup in this way: (Please comment on everything so that I may learn and improve my design.)

App_Code folder - contains the xsd files

App_Data folder - contains the mdf database file

ddlClientName - drop down list for client names

ddlPOBox - drop down list for po box numbers

 

ddlClientName_SelectedIndexChange event

 protected void ddlClientName_SelectedIndexChange(object sender, EventArgs e)
{
string strConn = @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\mail.mdf;Integr ...