f[x_,y_]:=2-x^2-y^2
graf=Plot3D[f[x,y],{x,-2,2},{y,-2,2},BoxRatios->{1,1,1}]

D[f[x,y],x]
D[f[x,y],y]
D[f[x,y],{x,2}]
D[f[x,y],{y,2}]
D[f[x,y],{x,2},y]