If you are registered under service tax, then you will have to login to your account at http://www.aces.gov.in to receive the provisional id and password. Enrollment on GST common portal for service tax payers will start from 1st January, 2017.
The respective State VAT/Service Tax department will be communicating the Provisional ID and Password to the respective registered taxpayers. Taxpayers registered with service tax/excise but not registered with VAT can get thier provisional ID and password by signing in to ACES.GOV.IN website.
VAT registered assessee can get their provisional ID and password from State VAT department website by signing in to the VAT portal