The entire blog world keeps talking about how it's not that hard to be a professional blogger. Usually the people saying that are the ones who have something to sell you, quite frankly. If someone's telling you this is not that difficult, you should look at what their motive is. Do they have something to sell you so that you'll become inspired, you'll think you can do it, or you'll start building a blog?
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.