Sales taxes in QuickBooks Online are calculated based on the tax code you assign, taxable items, customer tax status, and the amounts on the transactions. To get started with this, you'll need to set up your GST by following the steps in this article: Setting up QuickBooks for GST.
You can also use these for references when setting up other tax codes:
After setting up the sales tax, you can already create invoices that are calculated with GST. Make sure it is GST-read invoices. You can follow the articles on how to do this:
Once you're done creating invoices, the sales taxes will now flow on your reports. There are different tax reports that you can pull up in QuickBooks Online. They are listed in this article: What are the different tax reports available in QuickBooks.
You might want to know more about the tax reports by browsing these links: