One way:
You could do it with an event macro.
Something like this could get you started:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Range("A1") = "" Then
MsgBox "Type a value in cell A1", vbInformation, "Value Missing"
End If
End Sub

This way of doing it will be fantastically annoying for anyone who leaves
the cell (in this case A1) blank.

To use this, right-click on the sheet tab you want the message to appear for
and choose View Code.
There will be a Drop-down with the word General in it. Change it to Worksheet.
Paste this:

If Range("A1") = "" Then
MsgBox "Type a value in cell A1", vbInformation, "Value Missing"
End If

into the middle of the lines that are already there.

If you aren't familiar with macros, check out:
http://www.mvps.org/dmcritchie/excel/getstarted.htm

tj

"sugarbrit17" wrote:

I am trying to create a spreadsheet where I require the user to enter data
into a certain cell. If they do not enter data in this cell I would like a
warning to pop up to remind them