you need TF-IDF only if you have the actual contents of the blog, i.e. text. In this case you can find some useful video tutorials on text mining here: http://vancouverdata.blogspot.de/2010/11/text-analytics-with-rapidminer-loading.html
I would first focus on the text and add the other attributes like author and date later on. If you need help feel free to come back to this forum.