You can sign up for a free account by clicking the Get Started button on the homepage. Once you have an account, you can add your books, set up the pages and audiobook length, fine tune them, and keep track of your listening/reading progress.
An account allows you to save your books, view and update your progress across devices, and get a progress timeline.
When adding a book, you can set the actual starting page. This will be used to calculate your progress. If you don't know the actual starting page, you can set it to 0, but this will throw off the calculation.
When adding an audiobook, you can set the actual starting time. This will be used to calculate your progress. If you don't know the actual starting time, you can set it to 0, but this will throw off the calculation.