Gst nill file error

Hello mam,
I started my business from 1 september and i registered gst at 5 sep for e-commerce.
Still i have 2 bills of 15 oct 2017.
And when i fill it and people are saying first fill nill file of july but in my gst panel showing September, oct, nov.
Please tell me some thing. When should i file return. And what shuld i do.