You'll need to be approved for a developer token before you can begin using the APIs. Or you can set up a test account, which will let you execute API requests, but cannot serve ads or retrieve specific data. The links Danial posted should help you do that.
Each client library has an examples section you can refer to. Fill in the relevant variables before running the file, and it should work. You can adapt the examples to your needs, or use them as a reference.
Here are the PHP client library examples, you can find similar github repos for many popular languages.
Hope this helps,
Neil