create a dropdownlist and add items to the list and set the value to each item to the url of the page you want to fetch
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True">
<asp:ListItem Value="http://www.something.com/page1.htm">Item1</asp:ListItem>
<asp:ListItem Value="http://www.something.com/page2.htm">Item2</asp:ListItem>
</asp:DropDownList>
then when the dropdownlist changes item it redirects to the desired url
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
Response.Redirect(DropDownList1.SelectedValue);
}