The earth is a sphere? If you mean on a map, then the point on the equator directly south of Greenwich is the centre, which is in the sea below West Africa. Why Grenwich? The map was made when the British empire was around
Andrew J. Woods, a physicist at Gulf Energy and Environmental Systems in San Diego, California, used a digital spherical map and calculated the Earth's center coordinates as 39°00'N 34°00'E on a mainframe system.
Sorry to tell you, but using everyone else's defintion of centre, not some weird mathematical defintion the centre of earth is in the centre(newtonian centre)