Is GST registration is mandatory for online business for beginners

yes , for online business GST Registration mandatory.

Yes... Required Registration under GST.

Yes, gst registration is mandatory for online business.

As per Section 24(ix) of the CGST Act, 2017,
It's mandatory to obtain Registration

But
No ,..As per Notification No. 65/2017-Central Tax dated 15th November 2017 has exempted them from Registration if they do not exceed the threshold limit as per Section 22(1) of the CGST Act 2017. (20lac)

So you are exempted to obtain Registration unless unless your turnover exceeds 20lac

You can take E-commerce registration..