The potential is constant inside the sphere as the electric field is zero inside the sphere. So, the value is given by 

$V=\frac{Q}{4\pi {\epsilon }_{0}R}$ for $r<R$

The potential at the surface is 

$V=\frac{Q}{4\pi {\epsilon }_{0}R}$

The potential outside the sphere starts decreasing and is given by the formula 

$V=\frac{Q}{4\pi {\epsilon }_{0}r}$ for $r>R$