Please, someone can let me know how to create new categories for my products (products on sale).

Hello there, @Andi22.

Let's go to the Products and Services page so you can create new categories for your products in QuickBooks Online (QBO).

You can use categories to group items of the same type. Doing so will give you helpful insights into your sales and inventory when running your reports in QBO. To create new categories, follow these steps:

1. Go to Get paid & pay or Sales, then select Products and services.
2. Select the More dropdown, then choose Manage categories. 
3. Click the New category button, then enter the name of the category. Note: If you need a create a sub-category, select Is a sub-category checkbox and the main category.
4. Select Save.

Once you're done, add a new or an existing product or service to a category. For the complete guide, please see this article: Group your products and services into different categories.

Also, you may want to check out this article as your reference to guide you on how to use reports to get helpful insights on the things you buy and sell and the status of your inventory in QBO: Use reports to see your sales and inventory status.

Let me know if you have other concerns about product categories and questions about managing transactions in QBO. I'm always ready to help. Take care, and have a great day, @Andi22.