If you only want it to "appear" to be negative:
Format/Cells/Number/Custom -General;-General;0;@
Note that positive values displayed with the leading minus sign will not
be treated as negative in calculations.
If you want to have the values automatically be negative, try this event
macro (right-click the worksheet tab and choose View Code):
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
With Target
If .Count > 1 Then Exit Sub
If .Address(False, False) = "A1" Then
If IsNumeric(.Value) Then
Application.EnableEvents = False
.Value = -Abs(.Value)
Application.EnableEvents = True
End If
End If
End With
End Sub