Jody, no problem being self taught, you'd be surprised at how many people here are self taught. This is a great place to learn.
A formula you say. Humm that may be a tough one. You could set up Sheet2 as a log of all invoices. Put the invoice number in Column A and details in other columns. Then put in I2 on sheet1. =COUNT(Sheet2!A:A) Assuming your first invoice is #1. If you have alpha numeric numbering try something like ="B"&COUNT(Sheet1!A:A)+100 You would need to update the log sheet after completing each invoice. This is all extra work. But macros could be written to automate it.
Setting the invoice number by a macro is easy to set up. Press Alt + F11 at the same time or right click the sheet tab and select view code.
There are different types of modules in which to place code. Workbook which will activate a macro at events like open, close, save. worksheet modules which will hold macros for events like sheet change, Activate, etc. and general modules for other macros.
Now that you are in the VBA editor you will see a screen on the left hand side called Project-VBA project, if this is not visible press Ctrl + R
Doubleclick on Workbook icon from the Project screen.
A window will open on the right
Paste this code into the window
Code:
Private Sub Workbook_Open()
Range("I2").Value = Range("I2").Value + 1
End Sub
This will cause I2 to increase by one everytime the file is opened.
There is two dropdown boxes at the top of this screen.
Select the first bow and select workbook
go to the second dropdown box and you will see many other events that can be coded.
Hope this helps.