This is an important one. If you don't know who your audience is, you don't know whether or not the content you're creating is truly having a positive impact in your niche. Sure, you can rely on keyword research and analytics to steer you in the right direction, and you should, but you should also consider forming a deep understanding of who your audience is, what they want and what problems they're facing.
Perhaps the most popular network for placing these types of ads is Google AdSense. With this program, you do not need to be in direct contact with advertisers; you simply place the banner on your site, Google chooses ads relevant to your content, and your viewers click on the ads. There are countless similar programs available if you find that AdSense doesn’t work for you, such as Chitika, Infolinks, and Media.net.