It is a very simple idea, when you join Google Adsense program you will be given a special code that can be added in your website pages. This code will generate ads that are related to the content of your pages automatically. When a visitor clicks on one of those ads, you will get paid.
You need to understand the brother of Google Adsense that called Google Adwords, where advertisers want to market their products and services, and want to pay for every click. In fact, you're publishing those Adwords ads and you'll get the money from the advertiser who joined the Google Adwords program.
How to start:
First, you need a website or a free blog, I'd recommend you to start a free blog at Blogger.com. After you create a blog, start writing about 10 posts about things you know about. Anything will be working with Adsense. Try to write text rich posts as Google search engine and other search engines like the rich text content. After that, submit your blog to all blog directories located in this list. Then you need to update your blog in a regular base, at least every week.
When you satisfy with your blog, you can now request an account from Google Adsense from this link:
https://www.google.com/adsense/ and wait for one or tow weeks before they reply you. After they reply you, you can enter your account and generate the code that can be added in your blog.
Adsense Revenue sharing sites:
With these sites, you don't need a website to start earning from Google Adsense, you can just make an account with them and start writing= content and you will take a very good percentage from their Adsense earnings. Most of them will give you at least %50 from their Adsense revenue.
Adsense revenue sharing sites:
www.HubPages.com, gives you %60 "My Recommended"
www.HowToDoThings.com, gives you %50