To start off with, you would need to sign up for access to the API. While your application is in the process of being approved, you can setup the environment to make their first API call against test accounts. This would include setting up the PHP library for development and generating OAuth credentials for the AdWords account. Lastly, you can go through the sample code examples to get a heads up on using the API. Once your application is approved, you can start making API calls against production AdWords account, the process for which is the same as test accounts. To extract data from the API, I would recommend using the AdWords API Reporting. You can choose, from the list of reports, ones most suitable to your requirement.
Hope this helps. Please feel free to revert in case you have more questions.
Regards,
Vishal, AdWords API Team