The path to a profitable blog starts with choosing the best monetization method. While selling ads yourself can be quite lucrative, sometimes it’s best to utilize an existing ad network.
There are dozens of great ad networks out there, and each has their own strengths and weaknesses. You should take your time to research your options and select the best fitting ad network for your needs. Some ad networks specialize in text links, banners, mini malls, and others are solely commission-based.
Most networks are relatively simple to join; with only minor site and geographic requirements.
For example, one can apply for Google Adsense (by far the most popular ad network) and get approved in as few as 1-2 business days - https://www.google.com/adsense/
Once you find right ad network for your needs, simply place the ad code on your blog and track the results. It is always a good idea to try different ad networks as well as ad sizes, colors, and placements. A small change in how you present your ads can make a huge impact on your blog’s bottom-line.

Other ad networks to consider:

Text Links:
VibrantMedia.com
PaidTextLinks.com
Adbrite.com
TextLinkBrokers.com
Kontera.com
ValueClickMedia.com
Publisher.Yahoo.com

Banner Ads:
Casalemedia.com/Publishers
TribalFusion.com
DoubleClick.com

RSS Feeds:
Pheedo.com
FeedBurner.com

Related reading: Monetizing Your Blog With broadKast.me