How can I create a fictitious company for training, which won't affect my books?
Learning your way around the QuickBooks Online (QBO) system is my priority, @abmouton-gmail-c. That's why I've come to guide you how.
The QBO system offers a Free 30-day trial so you can get to practice your expertise in managing a business and act as the bookkeeper/accountant. By the name itself, you'll get to use the account for free within 30 days and then you'll be prompted to subscribe.
Choose the product you want to use (Simple Start, Essentials, or Plus).
Enter all the details needed (i.e., Email address (User ID), First and Last name, Phone, and Password).
Click the Start Free Trial button.
Follow the on-screen instructions to complete the process of setting up your company file.
Once you're done, you can get started with QBO. To learn more on the next steps you need in setting up your trial account (i.e., linking bank accounts and importing your lists), I'd recommend checking out this article: Get started with QuickBooks Online.
Let me know how it goes in the comments below. If you have other concerns in setting up your QBO account, I'll gladly help. Take care, and have a good one.