Do you see the little envelope that appears next to a poster's name when you click on their post?

Well, click on that envelope and a message box will appear to allow you to write to that person.

Remember, if that person is not VIP, he will never get your letter or even know that you tried to reach them.  Providers with links to their reviews all have PM privledges, by the way.

When someone sends you an email, you will see a notice of same when you log on.  Either click on that notice or hit the mail icon to get to your PM site where you can file away incoming and outgoing PMs just like on any email system.

If you click on a post, and do not see an envelope, that means that the person is using an alias.  That's the only fool proof way to know that the poster is using an alias, in fact.

...though the person's VIP membership has expired, so they won't get your PM until they renew. Don't be disappointed if you don't get a reply.

Newbie Manual where you can get all the acronyms decoded for you.  You should also read several pages here as many questions repeat often.