Hey, Kunal! This is an interesting question, I haven't heard too many cases where people get these two sounds mixed up, but I'll try to explain as best as I can. For "z", all you need to do is pronounce "s" and then add some voice! For "j", you are looking at a voiced, affricative sound. Can you hear the "ch" sound (i.e. itch, child, etc?) If so, all you need to do is say that sound and add voicing, and you'll have the "j" sound in English. You can actually try to say "Jesus", which has both the J and the Z sound. J at the beginning and Z in the middle. Let me know how it goes!