If you want to make money online via Google Adsense, you can apply for it free of charge but there guidelines you must follow for your application to be approved. Savvy publishers like you can avoid common application pitfalls by being aware of the trouble areas while also making sure you avoid forbidden content on your blog/website.
If your application has not been approved, please refer first to the application status message that Google sent to you - it contains more details on why your application was disapproved. Below, are some of the most common disapproval reasons and the next steps you should follow.
==> There was a problem with my contact details or name
Please sign in under www.google.com/adsense with the email address that you first applied with. On the next page, you will see a form where you can correct your address data and resubmit the application.
==> There was a problem with my website content
Please review Adsense guidelines to make sure that your website complies with Adsense policies. After you have adjusted the content of your website, you can resubmit your application under www.google.com/adsense.
==> The message says that I already have an account under another email address
At this moment, AdSense policies only allow one account per publisher. Please refer to the email that Google sent to you and follow the steps outlined there to either keep using your existing accounts or move to a new account. Make sure that you follow the instructions in the email rather than just closing your old account, as doing so will require you to submit a new application with a different email address.
You will probably face this kind of scenario when another Adsense publisher is a full admin on your blog which you also access with the email you used to apply for Adsense. If this is the case, remove the person from your blog and re-apply with a new email address at www.google.com/adsense.
NOTE: Publishers whose accounts have been terminated for invalid activity or policy violations will not be allowed to open a new AdSense account according to the AdSense T&Cs.
Source: Adsense Help