param = {z0 -> 0, v0 -> 0, g -> 9.81 , k -> 0.2};
p1 = Plot[{z[t] /. param}, {t, 0, 20}, 
  PlotStyle -> {RGBColor[0, 160, 1]}, PlotLabel -> "z[t]", 
  AxesLabel -> {t, z}]
p2 = Plot[{vz[t] /. param}, {t, 0, 20}, PlotRange -> All, 
  PlotStyle -> {RGBColor[33, 0, 1]}, PlotLabel -> "vz[t]", 
  AxesLabel -> {t, vz}]

something like that need to give the parameters i need to make it step by step first -20 -10 10 then 20 then at last Show param or something like that

Something like this?

BarChart[{-20, -10, 10, 20}, LabelingFunction -> Center, AxesLabel -> {"x", "y"}, LabelStyle -> Directive[Medium, Bold], BarSpacing -> None]

Cheers,

Marco