You will need to register with GST as first action and then you need to mention the commencement date in registration. You wil get 20 lakhs as exemption, the amount above 20 lakhs you will have to pay GST.

The is an interest of 1.5% per month on the amount liable to pay and also late filing fees of 100 per months.

It all depends on whether you were required to file Nil return or taxable return. If you need further assistance or consultation feel free to consult me at canamanmaloo@gmail.com