There are two ways to go about this -- sell your own ad space, which is time consuming or use a third party ad broker to do all the selling for you
1. Join an advertising network. Advertising networks are organizations that aggregate Web sites that offer advertising space, and sell banner ads (and other advertising options) across them. Traffic requirement is often a minimum of 5,000 monthly impressions, although many do not accept sites hosted by free page services.
Burst Media http://www.burstmedia.com
Fastclick (ValueClick). http://www.valueclickmedia.com
Advertising.com. http://www.advertising.com
Tribal Fusion http://www.tribalfusion.com
2. Participate in contextual advertising networks. Contextual advertising are text ads delivered based on the content of the web page using an automated system.
Google Adsense https://www.google.com/adsense/
Yahoo! Publishing Network (currently in beta and available to US publishers only) http://publisher.yahoo.com/
Intellixt http://vibrantmedia.com/site/web_01a5.htm
Quigo Adsonar http://www.quigo.com/adsonarexchange.htm
IndustryBrains http://www.industrybrains.com
Kontera http://www.kontera.com
Kanoodle Brightads http://www.kanoodle.com/about/brightads.html
Clicksor http://www.clicksor.com/affiliate_programs.php
Contextweb http://www.contextweb.com/
Bidclix http://www.bidclix.com/
Bidvertiser http://www.bidvertiser.com/
If you are just starting to earn from advertising, it is easier to start using any of the above ad networks. Some of their advertisers may notice you -- and then skip the ad network and advertise on your site directly. Or at the very least, you get to understand the process and what types of ads work well with your website so you know who you will target