What Norie means is:
(1) Press Alt+F11. (Opens the VB Editor)
(2) From the menu choose Insert & UserForm
You will now have a "blank slate" of a userform to which you can add your controls. - (from the toolbox - If it's not showing choose View & Toolbox)
You can add code to the controls as you go.
UserForms in general are not that hard to learn but there are quite a few little steps to take and options available to you so they can seem rather daunting at first.
And oh yeah. (just for fun) to answer the question your signature quote asks, try something like this in your sheet code module.
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Columns(6)) Is Nothing Then Exit Sub
If Target.Count > 1 Then Exit Sub
If Target.Value > 0 Then
Application.EnableEvents = False
hold = .Value
.Value = -1
Target.PasteSpecial Paste:=xlValues, Operation:=xlMultiply
.Value = hold
.CutCopyMode = False
.EnableEvents = True