How to Choose Your Blog’s Topic

The obvious place to start with your blogging topic is the things that you are passionate about. If you aren’t interested in something but choose the topic because you think it meets the other criteria, you’re either going to stop writing or simply create a bad, dispassionate blog. Either way, it’s not worth choosing a topic just because it’s “easy”.
Once you’ve narrowed the field a bit to things you can actually write about, you need to look for a topic that appears to be decidedly under-served. An area where there is a group of people who either don’t have a blog (or at least not a good one) that caters to a particular interest they have.
The easiest way to do this, when starting out, is to target a niche within a niche, within a niche.
What this means is that, when picking your topic, you first look at a broad area of interest, then choose a subsection of that interest, and then choose yet another subset of that particular interest. This way, you’ve started with an area of very broad interest and drilled down until you have a tiny section of it that, while still having an audience, is likely greatly under-served.
A quick example of this works as follows:

1. Start with a Broad, Already-Popular Product: Apple products
2. Choose a Segment of that Topic: Apple Mobile Products (iPhone, iPad, etc.)
3. Choose Yet Another Segment of That: Apple Mobile Products for Students (College, High School, etc.)

What you have then is a blog about Apple’s mobile products aimed at college students. It could cover, at least in theory, apps for academia, how new features may help students with the academic/social life and rumors about Apple deals aimed at college students.
All in all, if you use that formula, you’ll usually stumble upon topic ideas that others have not tried, or at least not tried well. After several searches on the above topic, I was not able to easily locate an entire blog aimed at this market, just many static resources and individual blog posts.
The idea is that A) The topic has an audience, albeit not a huge one or an aware one and B) Once you become established in it, that you can use your reputation within this niche to expand, slowly, into the higher ones, eventually talking more about mobile Apple products in general and then Apple products at large.
However, the work isn’t quite done at this point. Now you have to make sure that there’s enough to write about and that you’re interested enough to keep writing on the topic. The best way to do that is to simply write. Try to get at least a month’s worth of posts done before taking the blog live, both to make sure there’s enough interest on your part and to ensure that there’s enough of interest to keep the blog active and interesting.
Once you do that, you most likely have a good topic that you can start your blog with and one you can grow on as well, setting the stage at least for a very successful blog.