Hey!
The best place to start would be the documentation. It has a getting started guide, reference materials, and a link to our Developer Agreement, which is important to abide by as a developer. Otherwise, you create a developer application at the page you linked above and use that information to call the API.
Don’t worry too much about the information you provide on that page. You can change it after you’ve created the application. Just make sure the redirect URI is a server that you control. While developing, you can use localhost if that is where you’re developing.
Hope that helps!