phi[x_,y_]=x y

Plot3D[phi[a,b],{a,-3,3},{b,-4,4}]

plot1=ContourPlot[phi[x,y],{x,-3,3},{y,-4,4},ContourShading->None,Contours->50]

anyvector={1,2,3}

gradientfield[x_,y_]={y,x}

gradientfield[4,5]

vectorplot=VectorPlot[gradientfield[x,y],{x,-3,3},{y,-4,4}]

Show[plot1,vectorplot]