Everyone knows the basic formula for the TV business: Networks pick the shows they believe will be the most popular and make money by selling commercials during and in between those shows. The better the show, the larger the audience. The larger the audience, the more they can charge the advertisers, and the more money they make. Their success or failure depends on the quality of their shows.
Likewise, if you want to generate revenue from online ads that you place on your site, the success of that effort is going to depend on the quality of your content. The better the content, and the more in demand it is, the better chance you'll have to make some money. Ads that you place on your site will generate more revenue as more people see them, which means that your goal will be to get a large audience to your site, keep them there as long as possible, and get them to visit as many pages as possible. The longer they stay, the more they see, the more ads are put in front of them, and the more you make. Simple, right? The problem is finding the right kind of content that's going to accomplish all of that.
Think carefully about the type of content you can put together that can grab an audience. It doesn't necessarily need to be original, self-made content (although that's the best way to guarantee an angle that's different from anything else out there). You can also collect and monetize other people's work—blogs, articles, videos—by gathering content that works well together. For example, suppose you want to run a sports news site. You can include all sorts of content produced by others that will attract a sports-minded audience and run advertising around it. Just make sure that you get permission from the content providers first, so you don't violate other people's copyrights.