Welcome to the blogosphere!
First off: picking a topic. Most blogs have a particular angle, but many are "life" blogs with some posts about a particular expertise thrown in. For example, you might write about your experiences with your children most days, but every once in a while devote an entry to new recipes (or have a regular "recipe Monday" or something like that). The great thing about blogs is that they're not sterile academic journals; everything is somehow related to something else so it's easy to focus on one thing, like education, while tying that in to something that you were thinking about, or read, or saw your children learning, etc. It's great fun.
Secondly: starting a blog is easy! Free sites such as blogger.com (as well as many others) allow you to sign up in minutes. They have several background styles for you to choose from and you can upload a photo for your header, etc. If you're not too computer savvy, I would start here and then later get a friend to help you if you want more bells and whistles (or hire a local computer kid to put one together to your specifications). Posting is super easy: if you can navigate forums like this one, then you'll have no trouble with blogger.com!
I have too many favorite blogs; two of my favorites are:www.nectarandlight.typepad.com/
(Wordpress and typepad are two other popular blogging/journaling sites; you may or may not have to pay for their services but it's worth checking out.)
Anyway, good luck and get blogging!