I don't understand how volt/m=C

Assuming from the context you mean V - volt (electric potential), m - meter (distance unit), C - coulomb (charge unit) - it is not, so whole discussion is a moot.

[tex]V = \frac {potential~~energy}{charge} = \frac J C = \frac {kg\times m^2}{C\times s^2} = \frac {kg\times m^2}{A\times s^3}[/tex]

V/m is an electric field unit.

Integrating the electric flux over a closed surface will yield electric charge inside the surface (Gauss law), so these are all closely related units and and ideas, but they don't produce the relationship you stated.