How can Superman walk on the Earth, if he defies Earth's gravity?

because he is a super hero. Super heroes can do almost everything and anything

He is like plane, they can fly but drive on the earth

It really depends on the story you chose. The original origin of Superman had him as made of denser material than humans. This account for his resilience(Not hurt easily) and if he is more dense gravity actually has a stronger pull on Superman than a human, since the force of gravity is a relationship of mass and distance. The original Superman could only leap tall buildings, not actually fly. Later on, Around the time of Crisis on Infinite Earths or a little later I think, Superman's powers were redefined as merely being a force of will. He can do it because he believes he can. Any more recent than that I really don;t know.

Flying mean zero gravity, there is impossible zero gravity on earth. He look like float but not flying like bird. Actually i think the super hero really look stupid but i enjoy the story.

Superman is the Man of STEEL, and we all know the earth has a magnetic core.

He can stop bullets from a gun, he can Jump tall buildings, He is faster than a speeding Train, He is SUPERMAN.

So the magnetic core holds the Man of steel to this planet.

Everyone knows that, OOOP's you didn't. haha