Is selenium supplement good as for antioxidant purposes in our body. and what is the recommended dose for 24y male and is it good for liver .nash?

I wouldn't advise it: Thirty years of work on antioxidants has made it pretty clear that they don't prevent disease or improve longevity, despite all the theoretical reasons that they are supposed to. There's one reference in an obscure journal in 2005 to selenium having been tried for NASH and on the evidence it was a dismal failure -- if it worked, the research piranhas would be all over it. It's your choice.