Go to www.blogger.com or www.typepad.com (just to name two) and dive in. Their directions for setting up a blog are very clear.
You can write whatever you like. Some people use a blog to post their family news and photos and videos. Others participate in political blogging. Many teachers post their thoughts about their jobs and their students as they go through the school year. The types of blogs around the world are endless.
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You can explore the whole topic here.
what is a blog and how do you write one
2 answers
More:
http://www.ipl.org/div/blogs
http://www.ipl.org/div/blogs