The mistake that many people make when starting a blog nowadays - particularly if they intend to make money off it - is to write about a saturated niche. Blogs about making money from blogs, or internet marketing in general, are popular choices for the newbie. But there are squillions of those kinds of blogs already. You really are in for a lot of work before you begin to see any reasonable traffic with those subjects. If you are a newbie and want to see benefits fairly soon I would advise choosing a low competition niche if possible.
That doesn't mean that you should not write about the subject of internet marketing, say. It just means that you should find something specific within it to focus on. Maybe just write a blog about article marketing alone - or perhaps one about keyword selection. These are still competitive, but certainly not completely saturated. And being more focused content-wise you will crawl up the rankings quicker.
I've noticed just how competitive the whole internet marketing niche is through writing this blog. I started it quite a while ago now. And I've got a fair amount of content up. The traffic is slowly growing, but it's disappointing considering how much work I've put into it.
I'm not going to give up, since I'm very interested in this niche and am learning a lot. So I'll always have something to write about. But compared to other blogs I've got running, it's moving slowly. I started an arts blog only months ago now, for example. It's already drawing in more traffic than this one!