This is one of those things where some people have problems. The first question is how do I pick a niche. Then should I just look for a popular niche that seems to be making money. Well first of all you should pick a niche about something you like. It can be a hobby or just something you have a good interest in. The problem is if you just pick a popular niche then people may see that you just want to make money and you really don't care about the subject or your readers. That's when you will fight a losing battle unless your very lucky.
I have done that with my blogs I write. I try to pick things that I am interested in and know a good deal about them. I also try to pick things where I can help someone in some way. That for example is why I write this blog you are reading. If anything on it helps someone then I am doing the right thing. The ads for various items are there to give examples and if someone wants to click on something that is their choice.
Another thing that you can do is narrow down your niche. In other words break it down to a singular subject. There are too many times when we pick a niche and realize that there are too many paths to follow. The next thing you know is that you are not sure which road to take and you end up lost. Trying to write a blog for example on selling all kinds of items can be very difficult. You would be much better off to break it down to say for example electronic items and then break it down further to say for example televisions or ham radios. That way you narrow the subject and attract the people that are interested in your subject.
So get out there and pick a niche and narrow it down.