Type 1 in the first cell and 2 in the next cell
Select both cells
Drag the fill handle down to continue the series
Use the formula =ROW() for automatic serial numbers
Use the formula =ROW()-1 if your data starts from row 2
Use the formula =COUNTA($A$2:A2) to number only filled rows
Double-click the fill handle to fill serial numbers automatically
Copy the formula down using Ctrl+C and Ctrl+V
Use the Fill Series option from the Home tab
Convert the range to a table and use a serial number formula in the first column