Welcome to the developers forum!
If this is your first time hearing about it, check out http://api.pwned.com/docs and read our documentation. Get a little taste of how it it works.
The platform is build using the OAuth 1.0 protocol, so what this means is that you don't have to store credentials of users, thus taking a heavy burden off your shoulders.
If you've never worked with OAuth, I would recommend checking out http://oauth.net and read up on the spec.
First thing is first, create an application! http://api.pwned.com/newapp
Once you create an application, you will receive the Consumer Key and Consumer Secret. You will need these during the OAuth process.
As you may or may not know, there are 3 OAuth Endpoints in the authorization process, they are:
As you can see in the documentation, there are a grip of API endpoints that you can hit. There are some requirements for everything that might want to access, so make sure you read what they are.
If you want to access some of our public APIs, all you need to do is register and application and make a request with your consumer key and consumer secret.
As of right now, all of our data is being returned in JSON format, so keep that in mind while writing the response parser.
Side note, we do have an OOB method so you can authenticate users from desktop/mobile platforms. You must send me a PM requesting access to this. It is still in development, also we are taking a different approach on it, so it needs more testing.
Last, but not least, please ask questions if you get stuck or don't understand. We're here to help you out.