Crossing 20 lakh in the middle of the financial year

Dear Sir,

I offer online training course across India (interstate) and am likely to cross Rs 20 Lakh as my net turn over in couple of weeks. As we are nearly in the middle of this financial year, how should I approach GST. Should I get GST registration now? or should I get the GST registration in April 2020?

If I get the GST registration now then do I need to pay GST on the sales which happened in between April 2019 - till date say Aug 2019? Or I pay GST only for the sales which happen after the GST registration date which in my case may be Sep 2019? I will be thankful if you can explain and suggest to me in simple language.

Thanks & regards,

Shankar