When you must register
For services, the threshold is ₹20 lakh of turnover in a financial year, and ₹10 lakh in the special category states. Turnover means everything you invoiced, not your profit. Cross it and registration is compulsory within 30 days.
Below the threshold you may still register voluntarily. Some creators do, because larger brands and agencies are more comfortable with a GSTIN on the invoice.
The rate
18% on advertising and content services. It is added on top of your fee, not taken out of it. A ₹50,000 reel becomes ₹59,000 on the invoice, and the ₹9,000 is collected on the government's behalf, not earned.
The part that catches people out
Whether you charge CGST and SGST or IGST depends on where the brand is registered, not where you are. Same state as you: split it into CGST 9% and SGST 9%. Different state: one line of IGST at 18%. Get this wrong and the brand cannot claim the credit, which is the thing that gets an invoice sent back.
What has to be on the invoice
Rule 46 lists it: your name, address and GSTIN, a sequential invoice number, the date, the brand's name, address and GSTIN, the place of supply, a description of the service, the SAC code, the taxable value, the rate and amount of each tax, and your signature. An invoice missing any of these is not a tax invoice, whatever it says at the top.
After you register
Returns are monthly or quarterly depending on turnover, and they are due whether or not you invoiced anything that period. A nil return still has to be filed. This is the real cost of registering, and it is worth being sure you have crossed the threshold before you take it on.