Create script that auto-numbers the lines of a sales order sequentially?

I am okay with workflows, but not with scripting. I created a custom column field on my sales transactions. The intention of this field is to capture the numbering of the different lines of the sales order. For example, the first line of the sales order, I manually populate this new custom column field with the #1, to indicate that this is the first line of the sales order. The second line would then be 2 and so forth. Any way to set this to auto populate after I save/edit the record? Thanks