Google Adsense is a great way to make money. All you have to do is to have a google Ad sense account (which is free).
To get a adsense account you will have to create a website or a blog. If you are new and do not want to invest much then go to blogger.com or WordPress.com to set up your own blog. They are giving free service.
Your blog must have 6 months old. And if you have self hosted blog or site then make sure that your domain must have been registered 6 month back,Because Google has changed its policy and does not approve new site.
Your bog or site must have more than 5-6 good and original post. There should not be duplicate contents, otherwise Google will not approve.
Apply for Adsense Account (http://www.google.com/adsense/) if your blog is active and running. Don't apply for the Adsense account if your website/blog is empty. Just check your website thoroughly before applying to Google Adsense Account. If your website is blank or has very less or poor content, there is big chance that Google will reject your website.
You provide accurate information (like name, address, phone no etc) while applying because Adsense will check the ownership of domain through which you are applying for adsense account. They do this to check the fraudsters who apply with someone else’s domain.
Avoid applying for multiple accounts with same IP. Google has strict policy to approve only one account per IP.
Your site or blog must comply with Google adsense policies. Like illegal content, pornography etc. If your account is disapproved by Google then keep on applying. They will approve one day. Even my Adsense account got approved in third attempt.
Once you got approval for Adsense then you can optimize the Adsense contents. Adsense Help is best tutorial for this.