Google AdSense — Recommended ad network. Arguably the largest and most popular. There are other ad networks, but this is a good place to start.
Amazon Seller Account — Sell your products on Amazon. This can be an excellent program for you if you have a yen to go into online retail. The program includes various services you can choose to use such as fulfillment and promotion within the Amazon site.
eBay Seller Account — Similar to Amazon.
How to Attract Direct Advertisers — An excellent article that gives a good explanation of what you need to know if you want to attract direct advertising to your site.
Affiliate Programs — An excellent Forbes article that will educate you about the pros and cons of affiliate programs. It also has links to other helpful Forbes articles on this topic.