First, don't get conned by anyone telling you it's easy....it's not.
Second, it's a long road and you must have patience to succeed. Yes, you can make a niche blog and monetize it with affiliate ads, but it's going to take a very long time to create good content, then rank it, get good quality visitors and hope than some of them will buy through your links.
If there is a topic that you already enjoy writing about, then it's better to write on that topic and monetize it after you've built up an audience for your content. That way it seems less like work and you are more likely to stick to it, even if you don't earn anything at first.