Input credit on purchase

I am dealing in textile goods which was having zero rated tax prior to GST.

My registration under GST was applied for on 2.8.17 and GST registration was allotted on 16.08.17. Technically, I should have regd wef 1.7.17 but due to industry uncertainity things got delayed.  

However, I had purchased material with GST wef 22.7.17. Please enlighten:

a) how to avail input credit on GST paid material bought from 22.7.17.

b) I carry stock of pre GST zero rated tax.Am i liable to pay GST on such sale or is there any provision which will allow me to avail ITC. 

Thanks