Question Filling a grid

[Fuz]

I drew a grid with DrawLine of 5x5.
Now I should know how to fill a box of that grid (when there is clicked on), I know how to know where my mouse stands.
With

Dim MousePosition As Point
MousePosition = Cursor.Position

if i'm right?
But I don't know how to only fill the box that is clicked?
Can someone help me?

ps. A little request, can you keep it simple or give some additional information? As i'm just started to learn for a few months.

Thanks!