It doesn't matter whether you're a web savvy warblogger, a college student worried about assignments, or a retiree looking for a hobby, there are people out there with the same interests as you. Once you've decided you want to blog, the theme should be a natural progression. What are you passionate about? Is there enough to sustain regular updates? Some write on what's happening to them at the moment - this is truly a "web journal." Others focus on a primary theme, this attracts a loyal following, people with similiar interests who will come back for more. There are no "rules" to blogging, it's okay to deviate from your theme, just remember to be true to your blog. Keep in mind why you started your blog in the first place. Check out other blogs by googling your topic, find you niche then sit back and enjoy. The best advice?
Write to express, not impress.