You'll want to check out the
Authentication topic for a walkthrough of how the authentication mechanism works. That mechanism works by passing several tokens to the D2L Learning Environment to identify the App and the calling User. We offer an
SDK in several languages to manage this process for you. You can see this process in action using the
API Test Tool, which allows you to point to your target environment, use your own App ID\Key pair and authenticate with a user account. You can then make API calls against that environment. In fact, we just released
a blog post detailing several use cases for that sample.
Give those resources a try and then come back to the forums with any specific questions you have about your own development project.