In the declaration, add style="display:none;". So your div will look like:
<div id="someDiv" style="display:none;"></div>
This makes it hidden by default.
To show it, add following code in the button click:
<asp:Button ID="Button1" runat="server" OnClientClick="javascript:document.getElementById('someDiv').style.display='inline';" Text="Button" />
This will show the div back. Or if you are using jQuery, the syntax is
$('#someDive').show();
For showing scroll by default, check
css overflow property[
^]
Hope this helps!