NDVI calculations

How can I calculate NDVI from sentinel -1 GRD/ IW mode ?

I could be wrong - but at best you will be able to try and identify a correlation between backscatter and NDVI. NDVI is calculated using multispectral imagery - usually red and NIR bands and for that you will need Sentinel 2a imagery or similar like Landsat. SNAP has many vegetation indices already programmed in.