First of all know about GST, what is GST ?
GST - Goods & Service Tax
GSTIN -Goods and Services Tax Identification Number (15 Digit Number)
Go to the Government GST Portal (www.gst.gov.in) Service Tab then Registration Tab click - New Registration Tab
Open Form then Fill PAN, Mobile No., E-mail ID etc.
OTP (One Time Password) will be sent Email id & Mobile Both OTP will be Different.
Click Proceed Button (Part A completed)
You will receive a Temporary reference number on your Mobile and via E-mail after OTP verification
You will then need to fill Part-B of Form GST REG-01 duly signed (by DSC or EVC) and upload the required documents specified according to the business type
An acknowledgement with the Application Reference Number (ARN) will be generated in Form GST REG-02
More Detail Go to GST portal Web.