You need to migrate your existing service tax registration immediately and continue to pay gst.
If you did not have any such registration, then you need to apply within 30 days of crossing a turnover of ₹18 lakhs in any single financial year. The process of the registration is online and approval happens pretty quickly. The provisional registration is instantaneous, provided the servers work.
So, what I will suggest is - take a wise step and find out an attorney that can assist with the same. To this, I will suggest you LeadingFile. They are the best busines leading innovation...
Well, the entire process of GST registration may take -- 2 to 7 working days, subject to ROC, Govt & Client processing time.