registration

Basically two types of registration is there.
1.compulsory registration
2.voluntary registration
1.Compulsory registration : person who carries on any business at any place in India and turnover of such business exceeds thress hold limit then such person have to obtain compulsory registration within 30 days from he liable to get registered.
2. Voluntary registration : person who carries on any business at any place in India and turnover from business does not cross threshold limit inspite of that person registered himself as voluntary under gst then such person considered as taxable person and all provision of registered person similarly apply to him.

also there are some situation mentioned in section 24 for you you have to take compulsory registration without any limit refer sec 22 and 24 of cgst act 2017 for complete understanding