I'm not able to imagine how flat universe have infinite volume can anyone please tell?
"Flat" in this context doesn't mean flat like a sheet of paper, which is two dimensional. Space is three dimensional and thus can have volume; "flat" means that it doesn't have curvature. 3D curvature is hard (or impossible) to visualize, but it can exist, and we know from observations that on average the universe doesn't have any large scale curvature, so we call it flat. Not flat like a plane - flat like the regular 3D space we're used to.