Adding new columns within your Excel spreadsheet is super easy: Right Click, Insert, Columns.
And then I learned a new way recently and just shook my head. (Old dog, new trick syndrome...)
This works on Rows or Columns:
Click the row or column header (A,B,C.. or 1,2,3..)
Press CTRL + (plus sign) to add a column or row
Press CTRL - (minus sign) to delete a column or row