Hi Selene,
The following is an Event Macro, installation differs from
regular macros. To install: right-click on sheet tab, then
View Code, insert the following code.
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
If Target.Column <> 3 Then Exit Sub 'apply only to Column C
On Error goto ErrHandler
Application.EnableEvents = False
Target.Formula = UCase(Target.Formula)
ErrHandler:
Application.EnableEvents = True
End Sub
More information on Event procedures in
Worksheet Events and Workbook Events
http://www.mvps.org/dmcritchie/excel/event.htm
and specifically for Worksheet_change
http://www.mvps.org/dmcritchie/excel/event.htm#change
Other replies on use of UPPER Worksheet Function and
for validation do not match the request for an automatic change.
To change pre-existing entries for a selection with a regular macro, see
http://www.mvps.org/dmcritchie/excel/proper.htm#upper