It'd be great to think that you could simply build a site, sign up for a bunch of ad networks, and start making money. But unfortunately, it doesn't work like that. The ad networks have responsibilities to their clients (the advertisers) to place a certain number of ads within a given time period to the most appropriate audiences. It costs the ad networks money each time they place ads on a new site (they need to pay for accounting, project management, etc.), so the ad nets want to make sure they pick and choose the best sites possible for their brands. In other words, it doesn't happen overnight.
Start by getting your site and content together and using Google AdSense to place ads on your site. You probably won't make a lot of money, but something is better than nothing, and Google doesn't mind giving you ads during the early days when traffic is light. In the meantime, focus on methods to increase the traffic to your site. As you start getting more people on a regular basis, you'll be able to reach out to higher-paying ad nets with more confidence. In fact, if you get enough traffic coming on a regular basis, the ad nets will come to you.