GST liability

Your Liability will start from After Your registration...

As per section 22 of the CGST Act , 2017 registration is required if the aggregate turnover exceeds Rs 20 lacs.

If the application is submitted within 30 days from the date , when liable for registration, then the effective date of registration would be wheb liable for registration ( Rule 10) and would be able to claim ITC on stock preceeding that date,

otherwise,
effective date of registration would be the date of grant of registration

As per section 63 of the CGST Act, 2017 there exists provision regarding assessment of unregistered persons which calls for issue of SCN and levy of penalty as per section 73 and 74
of the CGST Act, 2017

You liable to take registration in this case. No needed to pay gst for past sales.