Adsense from Google would be a good start. If you get a little more advanced you could check out Commission Junction for affiliate advertising.
Rather than answer a question that has been asked numerous times throughout the forums here, I am just going to refer you to an honest answer just posted one forum down.
You can make money by placing ads on your blog. Other people will click on the ads and you make the money! Apply for Google Adsense program, get approved and place their ads.
One can earn through selling blogposts as well as blogrolls at their blogs too. But first you need to increase traffic at your blog. DON.
That's the million dollar WP question. I've tried to make money in all sorts of ways on wordpress, but the real key is to generate traffic. If you're getting less than 100 hits a day, you won't be able to make many affiliate sales or adsense clicks. Once you have the traffic you can get money through advertisements, adsense, or affiliate programs. Why not apply for an amazon affiliate license? The process is easy and free!
You can start by creating a good site. After that, you need to learn SEO, Marketing,Promotion, English, and how affiliate and ad programs work. Then you need to build traffic. Sounds easy right?
AdSense, ClickBank, CPA Lead, Chitika, Yahoo Publisher Network!. There's too many ways to properly count them all up!