See.
http://codex.wordpress.org/Function_Reference/get_comments
Set the orderby parameter to your desired value, possible values are.
comment_agent
comment_approved
comment_author
comment_author_email
comment_author_IP
comment_author_url
comment_content
comment_date
comment_date_gmt
comment_ID
comment_karma
comment_parent
comment_post_ID
comment_type
user_id
Refer to the link already provided for info on setting other parameters, and general examples.