When choosing a blog topic, the authors rely on 6 main criteria:
Your hobbies. Routine activities (under duress) can also be done at offline work. Excitement, passion, burning interest will help not to fade the desire to continue/improve your project. As an example, I will name "Games on paper" (bulls * cows, tic-tac-toe, sea battle, etc.), "Caring for a parrot" or "Growing strawberries." Describe your personal experience with a photo or video.
Own knowledge. You are a great mechanic who really loves his job. You can safely start the blog "Why the car does not start."
interests of Internet users. We are looking for answers to a variety of questions. Google Trends will help a little, or Some queries are temporary or seasonal, for example, "Sochi 2014 Olympics" or "New Year's gifts" (here you can see the dynamics of change). But the number of those interested in this issue in this period of time is disproportionately large, like, say, the income from the sale of flowers on March 8th. Such sites are created in advance, with a view to the future. After all, the trust of search engines comes gradually - it takes at least about six months.
Competition. For example, I chose the Blogger platform and not WordPress because of it. You can also develop a popular niche, but with a more specific site. Let's say, not "how to lose weight", but "how to lose weight for a guy." Here you can find the statistics of a set of words in Yandex.
A ceiling on the number of users who are interested in a given issue. Narrow-topic sites are easier to promote in the search engine, and users are more loyal to them. But here pretty soon you can reach the maximum number of visitors. Solution: either start a new resource or expand the subject. For example, "Blogger's Cheat Sheet" was originally only about the Blogger platform. Then it was followed by internal optimization and web page code. These are very large sections, but in general they are interconnected and the reader is somehow interested in them. And so that he does not get bored, I publish articles of these headings in turn.
There is such a thing as "profitable niches", where there are more advertisers. These are health, finance, real estate, cars, gifts and some others. In Google AdWords, on the right side of the panel, there is a "Columns" button, where we indicate the "Estimate CPC" checkbox (the cost per visitor clicked on an ad's link). The number of advertisers in Yandex by keywords can be viewed here.
Just pass by "adult topics" or against the law. There are specific features of the genre.