1. First, go to the first line you want to comment, press Ctrl-V. This will put the editor in the VISUAL BLOCK mode.
2. Then using the arrow key select until the last line
3. Press Shift-I, which will put the editor in INSERT mode and then press #. This will add a hash to the first line.
4. Press Esc (give it a second), and it will insert a # character on all other selected lines.

source: https://stackoverflow.com/questions/1676632/whats-a-quick-way-to-comment-uncomment-lines-in-vim