how to open a page in new tab on button click in asp.net?

You could use window.open. Like this:

protected void btnNewEntry_Click(object sender, EventArgs e)
{ 
   Page.ClientScript.RegisterStartupScript(
   this.GetType(),"OpenWindow","window.open('YourURL','_newtab');",true);
}

Why not just call window.open straight from OnClick?

<asp:Button ID="btnNewEntry" runat="Server" CssClass="button" Text="New Entry" OnClick="window.open('New.aspx')" />

Take care to reset target, otherwise all other calls like Response.Redirect will open in a new tab, which might be not what you want.

<asp:LinkButton OnClientClick="openInNewTab();" .../>

In javaScript:

<script type="text/javascript">
    function openInNewTab() {
        window.document.forms[0].target = '_blank'; 
        setTimeout(function () { window.document.forms[0].target = ''; }, 0);
    }
</script>

Use JavaScript for the main form / Button click event. An example is:

Context.Response.Write("<script language='javascript'>window.open('AccountsStmt.aspx?showledger=" & sledgerGrp & "','_newtab');</script>")