It sounds like you are asking how to send a PM (private message which only you and the recipient can see) on YouTube.
To send a PM to another YouTube user:
- Create a YouTube/Google+ account if you do not already have one.
- While exploring YouTube, click on the user you want to message. This
will momentarily cause more info about the user to hover above
the username but don't click again, just wait and you will be
directed to the user's channel page.
- Depending on that user's privacy settings, you may see the following links across the bottom of the Channel header:
Home Videos Playlists Channels Discussion About
- Click Send Message
- Enter your message and click Send
If the user you are trying to send the message to does not have a "confirmed" account, this feature will not be available in their channel. Your only alternative will be to arrange, initially via public comment, to communicate over another means. Keep in mind also, if their comment is rather old (a few years at least), they may have posted that comment before Google+ was melded with YouTube, in which case if they have not Created a Google+ account, they may be unable to even make new public YouTube comments. If their comments are more recent, you can obviously rule that out though.
Here is the official help page from Google Support.