Take Ajax Contol ModalPopup
<asp:button id="Button1" runat="server" text="PopUp" xmlns:asp="#unknown" />
<cc1:modalpopupextender id="modalpopupadd" runat="server" popupcontrolid="UpdatePanel1" backgroundcssclass="popupbodybg" xmlns:cc1="#unknown">
Drag="true" TargetControlID="Button1" CancelControlID="btnclose" CacheDynamicResults="True">
</cc1:modalpopupextender>
<asp:updatepanel id="UpdatePanel1" runat="server" updatemode="Conditional" xmlns:asp="#unknown">
<contenttemplate>
Write here which u want to pop up........
<asp:button id="btnclose" runat="server" text="Close" onclick="btnclose_Click" />
</contenttemplate>
</asp:updatepanel>
1> take one button name Button1
2> take one ModalpopupExtender. and set properties like above.
3> take updatepanel or panel which u want to popup.