Help starting student Blogs

I'm just trying to get my students started on creating their own blogs for our math class.  They have access to their own in-house blog though our district's use of StudyWiz. I introduced it the other day and had them create their first post.  However, when I came back the next day, they had placed over 25 comments on one student's blog about that evening's football game and who one of the students liked.

Does anyone have any suggestions or ideas on introducing blogs to my 6th graders?  Specifically, I'm looking for:

* rubrics to assess their blogs
* ways to keep them from using the comments as a chat room (I'm thinking the rubric may help with this)
* student-friendly tips for what makes a good blog