First off, you must apply for an account with Google Adsense.
Once approved by Adsense, you may apply, if you wish, for the Hubpages Ad program, and ebay.
You can also join amazon.com as an affiliate.
From there....you just write. Write what you know a lot about. Write what you are learning about. Write about what you'd like to own.
Write about your profession, your life, anything - if you write well, you can make some money. If you learn to research things with keywords and write well, and are prolific at it, you might make a pretty decent amount of money.
It's definitely not a FAST thing....in fact, it is rather slow going at first. This is more of an endurance game, I think.