Try this
Initially
<asp:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0"
Height="367px" Width="1000px">
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
if(TabContainer1.ActiveTabIndex==0)
{
}
else if(TabContainer1.ActiveTabIndex==1)
{
}
}
}