? There is nothing like that. A blog is a functionality that has to be coded in concordance with other parts of your site. I suppose you are a newbie in programming and in php. That's no problem, but you have a lot to learn.
Or simply download a blog engine, install and use. Look around here: http://www.thefreecountry.com/php/free-blog-scripts.shtml
Be aware, that these applications might vary in complexity and features. Thus you have to decide first what your requirements are, than find a suitable one, try to tailor an existing one to your needs, or write your won.