I think there are several types of blogs. Some are designed to reach a particular audience and enhance careers (e.g. Bonnie Hunter, most longarmers); some are designed to make money from the ads; and some are more like journals without the intent of generating financial gain. Decide what you want from your blog, and then proceed. You can always quit if you don't like it. If you intend to put in information that is very personal, you should consider putting restrictions on who can read your blog.