Welcome to the Board!
Here's a quick example that you might be able to fit to your needs:
<font face=Tahoma><SPAN style="color:#00007F">Private</SPAN> <SPAN style="color:#00007F">Sub</SPAN> Worksheet_Change(<SPAN style="color:#00007F">ByVal</SPAN> Target <SPAN style="color:#00007F">As</SPAN> Range)
<SPAN style="color:#00007F">Dim</SPAN> rng <SPAN style="color:#00007F">As</SPAN> Range
<SPAN style="color:#007F00">' Only look at single cell changes</SPAN>
<SPAN style="color:#00007F">If</SPAN> Target.Count > 1 <SPAN style="color:#00007F">Then</SPAN> <SPAN style="color:#00007F">Exit</SPAN> <SPAN style="color:#00007F">Sub</SPAN>
<SPAN style="color:#00007F">Set</SPAN> rng = Range("A:A")
<SPAN style="color:#007F00">' Only look at that range</SPAN>
<SPAN style="color:#00007F">If</SPAN> Intersect(Target, rng) <SPAN style="color:#00007F">Is</SPAN> <SPAN style="color:#00007F">Nothing</SPAN> <SPAN style="color:#00007F">Then</SPAN> <SPAN style="color:#00007F">Exit</SPAN> <SPAN style="color:#00007F">Sub</SPAN>
<SPAN style="color:#00007F">If</SPAN> Target = 1 Then _
MsgBox "Reminder message here", vbInformation + vbOKOnly, "Message"
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN></FONT>
The code goes in the worksheet specific module.
Hope that helps,
Smitty