To plot this in mathematica you will need to choose some values for the constants,

I would choose units for charge so that numerically

and we choose the radius of the sphere to be .

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.