Question

if the potential the surface of the Earth is assign zero then potential at the centre will be

Answer 1

Because of gauge symmetry, the 0 of potential can be chosen arbitrarily.  

If the Earth were constant density throughout, then V inside would be proportional to M(r)/r ~ r³/r = r².  

In fact, Earth's density increases with depth; i.e., decreases with increasing r, so that V decreases a bit faster with depth than r². Choosing V=0 at Earth's surface, will make V < 0 at Earth's center, with a magnitude that depends on the distribution of mass within the Earth, as Margaret sort of refers to; but as I've already pointed out, V continues to diminish all the way to the center.