Manage->Accounts->Account Types->Add
Name - Tax Account
Manage->Accounts->Accounts->Add
Name - Tax
Account Type - Tax Account
Manage->Accounts->Transaction Types->Add
Name - Tax Transaction
Source - Receivable Account
Target - Tax Account
Default Source - Receivables
Default Target - Tax
Manage->Products->Tax Templates->Add
Name - GST {or your tax name here}
Account Transaction - Tax Transaction
Rate - 10 {meaning 10%}
Click on Mappings and click Add. You can map to all products, or product groups or individual products, even departments.
Now when you select a product, the Tax will be shown at the bottom of the ticket screen.
Note : When using Taxes, you product price needs to be ex-tax.
PS - To remove this, delete the above settings in the reverse order. But once transactions have been made, accounts cannot be removed, so use the Mapping to disable the Tax Template