First read about Adsense and understand it. You can follow this introductory post here. Create free blog from Blogger or wordpress or create your own website and write some unique contents of your interest. Apply for Adsense and put the adsense code in your blog or website, then try to make your blog or website popular and increase traffic of your blog or website. Then you can make money from adsense.
First - get some visitors. Do this by making an orginal interesting blog that people want to read. Do not do what I see (some, but far too many) other people on DP doing, getting a free blog, using a standard template, putting some copied articles and a couple of videos off youtube on it and then complaining that you're not making money. You have to earn your adsense money, make a blog that advertisers will want to be on, make one that engages people so that they stay on it long enough to look around and see the ads.
To earn from adsense you must follow steps... 1. Make site or blog for you 2. Attract more and more people to your site and blog. 3. Add adsense account for your blog or site.. 4. Finally earn money online from adsense..