I have created the following package.

BeginPackage["AplicacionCartesiana`"]
AplicacionCartesiana::usage = "AplicacionCartesiana[f, {x0, x1, dx}, \
{y0, y1, dy}] plots the image of the Cartesian coordinates under the \
complex function f."
Begin["`Private`"]
AplicacionCartesiana[func_, {x0_, x1_, dx_}, {y0_, y1_, dy_}] :=
 Module[{xy, x, y, hg, vg},
  xy = func[x + I y];
  hg = Curves[xy, {x, x0, x1, dx}, {y, y0, y1}];
  vg = Curves[xy, {y, y0, y1, dy}, {x, x0, x1}];
  Show[Graphics[Join[hg, vg]],
   AspectRatio -> Automatic,
   Axes -> True]
  ]
Curves[xy_, spread_, bounds_] :=
 With[{curves = Table[{Re[xy], Im[xy]}, spread]},
  ParametricPlot[curves, bounds, DisplayFunction -> Identity][[1]]
  ]
End[]
EndPackage[]

But I want to know where I have to store it in my computer in order to use it (with Get[] or Needs[]) in other mathematica session.
Thanks.