This way
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title>Untitled Page</title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
<script type="text/javascript">
$(function () {
setTimeout(function () {
$("[id*=TextBox1]").focus();
}, 100);
});
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<cc1:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0">
<cc1:TabPanel ID="TabPanel1" runat="server" HeaderText="TabPanel1" ToolTip="Tooltip_TabPanel1">
<ContentTemplate>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</ContentTemplate>
</cc1:TabPanel>
<cc1:TabPanel ID="TabPanel2" runat="server" HeaderText="TabPanel2" ToolTip="Tooltip_TabPanel2">
<ContentTemplate>
Tab 2
</ContentTemplate>
</cc1:TabPanel>
<cc1:TabPanel ID="TabPanel3" runat="server" HeaderText="TabPanel3" ToolTip="Tooltip_TabPanel3">
<ContentTemplate>
Tab 3
</ContentTemplate>
</cc1:TabPanel>
</cc1:TabContainer>
</form>
</body>
</html>