Use the API Explorer to perform API operations on sample data. Once you've created a developer account you can get started right away. Your developer account comes with a QuickBooks Online Sandbox that is pre-loaded with sample data.
Just select a resource, pick an operation, set the request and response types, and perform your operations. To see the results with QuickBooks Online in your Sandbox company data, go to Tools>QuickBooks Sandboxes, and select your company.