Display ASP.Net AJAX Modal Popup on Page Load example ASP.Net

hi 

I used below code for show Welcome message in my first page of site index.aspx

  protected void Page_Load(object sender, EventArgs e)
    {


        string message = "Welcome to Our site.";
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        sb.Append("<script type = 'text/javascript'>");
        sb.Append("window.onload=function(){");
        sb.Append("alert('");
        sb.Append(message);
        sb.Append("')};");
        sb.Append("</script>");
        ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", sb.ToString());

I want when users come to site when it show index.aspx at first time it show popup menu that i define some control on it like TextBox,button,... and when users click on button, it go to index.aspx

How i can do it?

Thanks alot