A tax invoice is generally issued to charge the tax and pass on the input tax credit. A GST Invoice must have the following mandatory fields-

1. Invoice number and date
2. Customer name
3. Shipping and billing address
4. Customer and taxpayer’s GSTIN (if registered) **
5. Place of supply
6. SAC code
7. Item details i.e. description, quantity (number), unit (meter, kg etc.), total value
8. Taxable value and discounts
9. Rate and amount of taxes i.e. CGST/ SGST/ IGST
10. Signature of the supplier

Here is a sample invoice for your reference