i747m flashing rom

[anon(5561520)]

hey i'm a noob at this whole flashing thing, and i saw a dl for android 4.3 but for att and i flashed it on my rogers i747m s3 and i couldn't use data, is there any way to fix it or is there any custom rom for i747m based on 4.3 i can flash on to my phone
if there is please show me step by step how to do it pls and thank you i hate stock 4.1.2