Unfortunately, there is no single answer to this question since it can vary depending on what you hope to accomplish with the Google API. However, some helpful resources include the following:
The Google Developers site (https://developers.google.com/), which offers extensive documentation on all of Google's various APIs
The Google Cloud Platform Console (https://cloud.google.com/console/), where you can manage your project's resources and access the various Google APIs
The Google API Client Libraries (https://developers.google.com/api-client-library/), which offer official client libraries for many popular programming languages to help you get started using the APIs quickly and easily
