Have you ever heard of Kid Blog? It's a great tool for classroom blogs
Students have secure accounts and all of their posts have to be approved before they are seen on the internet. For first time blogging that's the comfort level that you might want.
Here is a copy of the blog planning sheet that a teacher I know used. This gave the students some basic ideas and suggestions for topics for blogging.
Teaching students how to make comments on a blog:
Ten Things your students can blog about:
Twenty Reasons why students should blog:
33 Ways to use blogs in your class: