Issues In finding the body tag in the source code of template to add the Javascript code

If you're using portal mode, I'd suggest NOT making this change as it could cause problems when upgrading. What you can do is in your master page in the template layout add a page load event and set the body css like this:

    <script runat="server"> 
        protected void Page_Load(object sender, EventArgs e)
        {
            Page.DataBind();
            CMS.DocumentEngine.DocumentContext.CurrentBodyClass += " yourBodyClass";
        }
    </script>

Is it javascript code or css you're trying to put into the body tag?

If it's javascript, you should put it into the header section, and there's a tab on each template where you can do that. If you want it to run on every page, do it in the master template, otherwise do it just on the template(s) for the pages you want it to run on.