Can there be anybody who understands how to add bit of code under every piece of joomla. Joomla version 1.5. Whitch files I ought to edit or possibly theres some wordpress plugin or component?

You may create a methodOrcontent material wordpress plugin that ought to focus on onPrepareContent trigger

function onPrepareContent($item, $params, $limitstart)
{
   // append text in $item->text
}

yet another way could be in template you override the vista of com_content and then add static text inside. Simple factor you can test is visit \components\com_content\views\article\tmpl\ pick default.php after that and set it in templates\template_name\html\com_content\article\

now here alter the default.php to contain that static text.

if you want some text/html (not php code), than this can be used wordpress plugin: http://extensions.joomla.org/extensions/style-a-design/articles-styling/7404 - it provides extensive configuration options