How do I add GST?

I need to add GST to our prices, and just want to make sure i am doing it right.

Do you mean you want to add Tax on your payment field? If so, once you have setup the products to your payment field, you can setup the tax of the products in the Tax tab found in the Payment Wizard.

For more information, you can check this article: How to Add Tax to Your Payment Form. If you are referring to a different concern, please provide us more details so we can further check your concern. 

Regards.