Select the column next to where you want the new column
Right-click the selected column header
Click Insert
A new column will appear to the left of the selected column
To add multiple columns, select the same number of existing columns first
Right-click the selection and click Insert
Use the Home tab, then click Insert, then Insert Sheet Columns
On a Mac, Control-click the column header and choose Insert
Keyboard shortcut on Windows: Ctrl + Shift + +
Keyboard shortcut on Mac: Command + Shift + +