The hardest thing to do with any new endeavor is to figure out how to get started. Before I started blogging I spent several months looking through class blogs from Australia, New Zealand and Great Britain. There were a lot more of those than ones in the United States at the time. I spend most of that time trying to make sense of design, options and content.
Before you ask your students to blog, you need the experience that can only come from blogging. You need to create your own. Actually, I think you should create and post on two different blogs.
The first blog you should create is a class blog. You really need to have the experience of creating something that represents your classroom, your students and yourself. You need to think carefully about the types of information you need to share, and just as importantly what you should not share. You need to think about blogging in an education context.
The second blog you should create is a passion blog. You should have the experience of writing about something you are absolutely passionate about. The entire focus of the blog should be about that one thing that makes you happier than anything else. You need to understand how important it is to be able to share something important to you, not just to someone else.
Then, after you have been writing posts for a couple months you should be ready to get your students started. Remember the lessons you have learned and share them liberally with your students. Don't forget the focus you have to make your education posts representative of the learning that happens in your classroom and also don't forget the joy you have when you are writing what you care so much about. Then you will truly understand why you need to have your students balance their blogs with content from your classroom and from their hearts.