Inserting text multi line

Press Esc to enter 'command mode'
Use Ctrl+V to enter visual block mode
Move Up/Down to select the columns of text in the lines you want to comment.
Then hit Shift+i and type the text you want to insert.
Then hit Esc, wait 1 second and the inserted text will appear on every line.

For further information and reading, check out "Inserting text in multiple lines" in the Vim Tips Wiki.