First way:

v -> select whatever needs to be commented out
:
(appears this) :'<,’>
add s/^/#
(it will be now :'<,’>s/^/#)
Press Enter

Second way:

For commenting out a block of text, do the following:

1. hit CTRL + v (visual block mode)
2. use the down arrow keys to select the lines you want (it won’t highlight everything)
3. Shift + i (capital I)
4. insert the caracter/text you want to add at the beginning of the line (e.g. #)
5. Press ESC.

To uncomment a block:

1. Go the to first line of code where you want to start uncommenting from.
2. Press 0 (To bring the cursor to the beginning of the line.)
3. CTRL + v and select lines to be uncommented.
4. x that will delete all the # characters vertically.

Source: https://www.quora.com/How-can-I-un-comment-a-block-of-text-in-Vim