Define VAT, GST or sales-tax rates once; choose them on items, invoice lines and bill lines.
Accounting → Tax Rates → New tax rate: Name * (VAT 20%), Code *, Rate (%) *, Description, Compound (applied on top of other taxes), Non-recoverable (cannot be reclaimed from the tax authority) and Active. Click Create tax rate.
Tax collected on sales accrues to the Tax Payable role account; tax on purchases to the tax-receivable side unless the rate is non-recoverable. The Sales tax liability summary report (Chapter 19) totals what you owe. Deactivate rates you no longer use rather than deleting them, so old documents keep their tax.