GST in five minutes, without the portal

If a quote says 10,000 + GST, the customer pays 11,800 at 18%. If a shelf price is GST-inclusive, the tax inside that 11,800 is 1,800, not 18% of 11,800.

Sell inside the same state: CGST + SGST, half each. Sell across a state border: IGST at the full rate.

When in doubt, look up the HSN. Calculators do arithmetic; classification is a lawyer-and-CA problem.