Help with starting blog please

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I've been thinking about starting a blog, but really have no idea how to go about it. I've read things about it being like an online diary, used for selling things (though I don't know how) and using it to write things that other people might be interested in. I don't know where to go to start a blog, or how you get people to look at it. Most importantly it must be free. Any tips would be great, or even a good explanation of how they work!