You have 3 options.
1) When you type in the data in the first cell, go to the cell where you want the date/time, and press Control-Shift-; Control-; Enter. The date/time will be entered.
2) If your data is in A2, and you want the date/time in B2, enter this formula in B2:
=IF(A2="","",IF(B2="",NOW(),B2))
Make sure to enable Iterative Calculation (File > Options > Formulas > Enable Iterative Calculation), and format the cell with a date/time format.
3) Use a macro. Open a copy of your workbook. On the sheet where you want the dates, right click the sheet tab on the bottom, select View Code, and enter this code:
Rich (BB code):
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column <> 1 Then Exit Sub
Target.Offset(, 1) = Now
End Sub
The first 1 in red means to monitor the first column (A), and the second 1 in blue says how many columns over do you want the date/time. Then close the VBA window and try entering something in column A.
Hope this helps!