Short answer
A GST tax invoice normally shows the seller's name, address and GSTIN, a unique invoice number and date, the customer's details, a description of each item with its HSN or SAC code, quantity, taxable value, the tax rate and the CGST and SGST or IGST amount, the total, the place of supply and a signature. The free GST invoice generator on this site lays this out and works the tax out, in your browser. Rules change, so confirm anything you are unsure of with your accountant.
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The checklist
Seller: name, address and GSTIN. Invoice: a unique number in a running series and the date. Customer: name and address, and their GSTIN if they are registered.
For each item: a description, the HSN code for goods or the SAC code for services, the quantity, the taxable value and the GST rate. Then the tax amounts, the invoice total (often with the amount in words), the place of supply and the authorised signature.
CGST and SGST, or IGST
If you and the place of supply are in the same state, the tax is split equally into CGST and SGST. If they are in different states, a single IGST at the full rate applies.
So an 18% sale inside one state shows 9% CGST and 9% SGST. The same sale to another state shows 18% IGST. The total tax is the same, and it goes to different accounts.
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Tax invoice or bill of supply
A tax invoice is used when you charge GST. A bill of supply is used when no GST is charged, for example by a seller who is not registered. The two look different because a bill of supply has no tax columns.
Which one you need depends on your registration and what you sell. If you are not sure, ask your accountant before you issue the first one.
Prices that already include GST
Many shops quote one price with GST inside it. The tax is then a part of that price, not added on top. To find the taxable value, divide the price by one plus the rate: a price of 1,180 at 18% has a taxable value of 1,000 and tax of 180.
Common mistakes
A repeated or missing invoice number, a mistyped GSTIN, the wrong place of supply (so the wrong tax head) and a rate applied to the wrong item are the usual ones. A GSTIN has a check character, so most typos can be caught before the invoice goes out.
Keep a copy of every invoice. The generator on this site remembers your business details on your device only, if you allow it, so you do not retype them each time.