What report should I use to prepare ST-1 Return (sales tax)?

It's nice to see you again here in the Community, @maciej_.

You can pull up the Sales by Customer Detail report in QuickBooks Online and customize it together with the sales tax details to show the information that you need.

Here's how:

1. Click on Reports at the left pane.
2. Look for Sales by Customer Detail on the search box. 
3. Enter the date range of the information that you wish to show.
4. Hit on the Customize button at the top. 
5. Tick on the Rows/Columns arrow.
6. From the choices on that page make sure that the Tax Amount, Taxable Amount, and Tax Name are selected. 
7. Select on Run Report at the bottom.

To learn more about the available reports in QuickBooks Online and on how to customize it, please see these links:

• Reports included in your QuickBooks Online subscription
• Customize your reports in QuickBooks Online

I got your back if there's anything else that you need. Just let me know by adding a reply below. Have a good one!