How to create a comment in Latex?

The fontawesome5 package offers the faComment command that results in a similar symbol to the one you show in your question. You could use it inside of a quote environment as the following MWE shows. If you want to use multiple similar comments, you could also define your own environment:

documentclass{article}
usepackage{fontawesome5}
usepackage{lipsum}

newenvironment{mycomment}{begin{quote}itshapefaCommentquad}{end{quote}}
begin{document}

lipsum[4-5]

begin{quote}
  itshape
  faComment 
  quad
  lipsum[3]
end{quote}

lipsum[4-6]

begin{mycomment}
  lipsum[3]
end{mycomment}

lipsum[4-5]

end{document}

Apart from faComment, fontawesome5 also defines some other related symbols: