How to obtain GST Number?
Any business generating a turnover above INR 40 lakh (the limit is INR 10 lakh for businesses operational in hill states and North-Eastern states) must mandatorily register itself under the GST laws as a taxable entity. This is known as the GST registration process. 
Once the registration is completed, the taxpayer receives a 15-digit number known as the Goods and Service Tax Identification Number (GSTIN) within three working days. 

Getting your GST Number online
Visit the GST website at https://www.gst.gov.in
Select ‘Taxpayers’ from the drop-down menu and click on 'Register Now’
Choose 'New Registration' and fill up the following details:
State and district where your business is located
Name of the business.
PAN of the business.
Email ID and mobile number 
Fill up the CAPTCHA code and click on 'Proceed'.
Enter the OTP sent to your email ID and mobile number and click on 'Proceed'.
You will receive a Temporary Reference Number (TRN) 
Log into the GST portal again and select 'Register' under the 'Taxpayers' menu.
Click on 'Temporary Reference Number (TRN)' and enter the details
Complete CAPTCHA and click on 'Proceed'.
Enter the OTP sent on your email ID and registered mobile number. Click on 'Proceed'.
You can check the status of your application and use the ‘Edit’ button to make any changes. At this stage, you also need to attach a few documents, such as

• Address proof for your business
• Photographs
• Bank details such as account number, bank name, bank branch, and IFSC code
• Authorisation form

Next, log into the 'Verification' page and review the declaration. You can submit your application by either of the following methods:
Using Electronic Verification Code (EVC) where a code will be sent to the registered mobile number.

By e-Sign method: Enter the OTP received on your phone number linked to your Aadhaar card.
Businesses also need to submit the Digital Signature Certificate (DSC) to digitally sign the form.
Once this is completed, you will receive a message on the screen intimating successful submission. You will also receive the Application Reference Number (ARN) on your registered mobile number and email ID.
You can track the status of your application on the GST portal using the ARN.
Once it is approved, you will receive the GSTIN on your email and via SMS. A temporary user name (your GSTIN number) and password are also provided to log in to the GST website. You can change the password and username for future use. 

Getting your GST number via Aadhar
From August, businesses are also allowed to obtain GST Number through Aadhaar authentication.
You can follow the same process stated above. When prompted for authentication, you can choose Aadhaar. A link is sent to your registered email ID and mobile number.
Click on the link, enter your Aadhaar number and select 'Validate'.
Once the server matches the details, you will receive an OTP on your registered mobile number and email ID.
Enter the OTP to complete the application. 

Conclusion
GST Number is provided free of cost and is useful for availing input tax credit when any goods or services are purchased. You also have an option to surrender your GST number.