A simple declaration for DropDownList can be done as below.

<asp:DropDownList ID="DropDownList1" runat="server">
</asp:DropDownList>

  If we want to add some items statically it can be added inside the Page as follows.

<asp:DropDownList ID="DropDownList1" runat="server">
<asp:ListItem>Select</asp:ListItem>
<asp:ListItem>1</asp:ListItem>
      <asp:ListItem>2</asp:ListItem>
      <asp:ListItem>3</asp:ListItem>
</asp:DropDownList>