By Default, all parts of the program will be Tax Inclusive and will use the default GST tax code.
If you wish to set new Sales Records to Tax Exclusive mode by default, you can go to Setup > Preferences > Misc and change the Default Prices As dropdown to Tax Exclusive.
This can also be done as needed on the Sales Record screen, via the selector at the bottom right of the item lines.
Editing/Creating Tax Codes
If you have tax codes other than the default, you may need to manually create or edit your tax codes.
To do this, go to Setup > Tax Codes, and review the tax codes and tax rates. Click a tax code to edit. Change the Tax Rate accordingly.
You can also set your default tax code by checking the Default Tax Code checkbox in the Tax Code details screen when editing a Tax Code.