There are several traffic-based ad networks you can apply to: BlogHer, Glam, or Lifetime Moms are just a few. It's rare that bloggers make much money from traffic-based ad networks (if you're looking for more than $100 you may be disappointed--and it's more likely you'll make less than $50 unless your traffic is stupendous). If you have a niche blog or a very popular blog, you may want to consider selling your own ads. It takes more effort on your part, but you can choose vendors/bloggers who fit your audience more completely.
If you do choose to go with a traffic-based ad program, read the contracts thoroughly to make sure you agree with everything. For instance one network doesn't allow you to do giveaways on a blog with their ads because it competes with their clients. Most ad networks will also ask you to sign a one-year contract and require at least one set of ads to be above the fold.
Contributing Editor at BlogHer.com (Web/Tech)
Owner, Blogging Basics 101
Co-author, TypePad for Dummies (due in stores Feb. 2010)