While income tax is charged on your income, GST is charged on most expenditure - on the goods that you buy (eg MP3 player) and the services that you pay for (eg getting your MP3 player repaired).
When you buy something from a shop or get a quote for a service, the GST should already be included in the price unless specifically stated otherwise.
The current rate of GST is 15%.
You'll pay GST on most things you buy.
If you're running a business you may need to register for GST and make payments to us. You may also be able to claim GST back from us.