a) Subject: It is much easier/faster/more rewarding if you blog about something you are an expert (or something that you are really interested in learning) and ADD VALUE to the community. If you want to blog for money, the same principles apply, but as somebody wrote here, you need to look for the sweet spot and target the largest audience/market possible.
b) Build an audience: Use your expertise and specially be CONSISTENT. You don't need to blog everyday but make it predictable (once a week, once a month, etc) if you want people to come back. Good content is better than SEO, but you only pick the fruits 1-2 years later as your work compounds.
Here are some blogs to read to get you started:
http://www.quicksprout.com/the-advanced-guide-to-content-mar...
My suggestion is to go for the blue ocean. Either blog in a sphere that is not crowded, or be significantly different than any existing player.
Easier said than done maybe, but have a goal in mind and strive to be the absolute best. I dont agree with the "just write".
If you can write good information posts based on information that has already been shared elsewhere, but do it in such a way that it becomes intelligible to a new audience, that's a solid and useful blogging approach.