The potential inside of a uniformly charged sphere of radius R is given by $\displaystyle \frac{Q}{4\pi \epsilon _{0}}\frac{1}{2R}(3-\frac{r^2}{R^2})$
Please help me on how to plot this equation in Mathematica
To plot this in mathematica you will need to choose some values for the constants,
I would choose units for charge so that numerically $\displaystyle Q=4\pi \epsilon_0$
and we choose the radius of the sphere to be$\displaystyle R=1$.
Since we only know the potential inside the sphere we should only plot from r=0 to r=R so the mathematica command you want to use is
Plot[0.5*(3-r^2), {r,0,1}]
and press shift and return together to evaluate it. The mathematica help should get you going with the syntax you need.