Making the selections in a dropdownbox dependent upon the selected item in another one is a pretty simple task. Here is a working demo.
<%@ Page Language="VB" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title>Cascading DropDownList Controls With AJAX</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ScriptManager
id="sm1"
Runat="server" />
<asp:UpdatePanel
id="UpdatePanel1"
Runat="server">
<ContentTemplate>
<asp:Label
id="lblTitle"
Text="Title:"
AssociatedControlID="ddlTitle"
Runat="server" />
<asp:DropDownList
id="ddlTitle"
DataSourceID="srcTitle"
DataTextField="Title"
DataValueField="Title"
AutoPostBack="true"
Runat="server" />
<asp:SqlDataSource
id="srcTitle"
ConnectionString='<%$ ConnectionStrings:MyDatabase %>'
SelectCommand="SELECT Title FROM Movies ORDER BY Title"
Runat="server" />
<br /><br />
<asp:Label
id="Label1"
Text="Director:"
AssociatedControlID="ddlDirector"
Runat="server" />
<asp:DropDownList
id="ddlDirector"
DataSourceID="srcDirector"
DataTextField="Director"
AutoPostBack="true"
Runat="server" />
<asp:SqlDataSource
id="srcDirector"
ConnectionString='<%$ ConnectionStrings:MyDatabase %>'
SelectCommand="SELECT Director FROM Movies WHERE Title=@Title ORDER BY Title"
Runat="server">
<SelectParameters>
<asp:ControlParameter Name="Title" ControlID="ddlTitle" />
</SelectParameters>
</asp:SqlDataSource>
</ContentTemplate>
</asp:UpdatePanel>
</div>
</form>
</body>
</html>