The first thing that you'll need to do as a teacher is to choose a blog platform. There's lots of them out there, and they all have their plusses and minuses. Personally, I've found Kidblog to be the easiest, safest, and best all-around blog platform for what I've been trying to do, so that's what I'm going to focus on here.
Next, you'll have to set up accounts. If you are using Kidblog, this screencast should help you out:
Once your accounts are set up, the next step is getting your students started. I recommend letting students choose a topic that they want to learn more about rather than choosing a topic for them. This allows them to take ownership of their blog right away.
For older elementary students, I try to stress that their topic should not be something on which they are already an expert, but something they want to learn about. This will allow us to discuss criteria for finding good sources of information, bias, and the importance of citing their sources. If they are already an expert (or think they are), they will tend to write from memory rather than doing research.
Whether Wikipedia is a valid source is always a hot topic among teachers. I like to discuss how wikis work with my students so that they understand that information on Wikipedia is a collaboration of information from people all over the world. For that reason, I don't discourage students from using Wikipedia as a tool to get an overview of a topic and to find valid sources for their topic by using the references at the bottom of articles. For example, I would not want them citing the Wikipedia article on Wallenpaupack Area School District, but would encourage them to use the references to get to the Pennsylvania Department of Education's graduation statistics page.
For citations, I like to use BibMe. I know there are multiple other citation tools on the web, but my students have found this one to be the easiest. For most websites, you can simply copy and paste the URL into the site and it will give you the citation in the format of your choosing. For our first blog posts, I require citations from at least two sources.
Finally, before letting kids start their posts, it's important to discuss the audience and purpose for which they are writing. Most students who have no blogging experience have only written for their previous teachers. Publishing information on the internet is not the same as turning in an assignment. Help them understand that they will be publishing information that others will be able to use to learn about their topic. Discuss how important accuracy, good grammar, and spelling are in order for the readers to believe in the credibility of the author. Talk about keeping bias out of their informational writing.
For younger students, allowing them to share what they already know is a great way to introduce them to blogging. This can be done as young as kindergarten. (Here's a great example of a kindergarten class blog.) Having a digital camera and adding pictures of student illustrations to their text can make this even more powerful. Starting with a sentence or two and an illustration is great. You want this to be a positive experience, and you want them to experience success. Talk about the importance of good spelling and grammar, but don't harp on it so much that students focus on that over content. As they write more and as they get more feedback in the form of comments, those things will improve.
One of the most important and powerful things you can do after students write their first posts is to publish them as far and wide as possible. We know that students need meaningful feedback in order to learn, and blog comments can be powerful, meaningful feedback Encourage parents, other classes to which you are connected, and anyone else you can reach to comment on your students' work. Teach students how to leave meaningful comments and let them comment on each other's work. ("I didn't know there were elephants in India. Thanks for sharing that information." rather than "Great Job, Suzie!!!!!!!!!!!!!")
Knowing that their writing is being read and appreciated by others will make the efforts they have put into their first post seem totally worthwhile. . And it will probably leave them asking you, "When can I write my next post?"