Make sure you have traffic and content on your website first
Content has to come first before any traffic will come to your website. I do not mean a few blog posts but I mean a large sum of good quality content. If your site has good quality content and has a good amount of traffic coming to it, your advertisers will come to ask you if they can advertise on your website. You do not even have to look for them anymore
If your site doesn’t have the traffic yet to get advertisers try to trade links, banners and advertising spots with other websites. It’s better to have your advertisement spots filled with free advertisers than with no advertisers at all. No potential advertiser likes to see “Advertise here” place holders. If no one advertises on your website, why would they advertise on your website?