There are many factors to consider when choosing a niche. I’ve written about many of them in this post – but for me a lot of it comes back to choosing something that you know about, that you feel you can say something useful and interesting on – ie what are YOU about?.
After that I’d take a good hard look at tools like Google Trends where you can track the popularity of different search terms over time to see if they’ve been trending up or down. Ideally you want to find a popular niche that’s growing so that you can position yourself for the future growth.
Other factors of course come into play including the number of other blogs and websites on a topic already (ie competition), whether the topic has monetization streams (if you’re doing it for money) etc – but you can read more aobut them in my post on choosing a niche for your blog.
PS: another useful tool in choosing niches is SEOBook’s Keyword tool which can help you think about the popularity of different keywords.