Type `1` in the first cell
Type `2` in the next cell
Select both cells
Drag the fill handle down to continue the series
Or type `1` in the first cell
In the next cell, enter `=A1+1`
Drag the formula down
Or use `=ROW()-1` if your data starts on row 2
Copy the formula down for all rows
Or select the range and use Fill > Series
Choose Rows or Columns
Set the step value to `1`
Click OK