1. Is GST always calculated at 15%?

Yes, in New Zealand GST is always 15% on all goods and all services.
In some countries there is a different percentage for different types of goods.

2. If petrol costs 98 cents a litre and I buy 10 litres do I round up the 98 cents to $1 first?

No! Work out the calculation first then round up the answer if necessary.