There are a number of ways to do this, however the easiest way is to find their name on a post (On the forum) or a comment (across the whole site) and click on their name. It will link to their profile page.
On the profile page, click Private Message. You can also add them to your friends, meaning that they remain on your profile which can be access anytime on the top right of the screen – where it says “Howdy…”
Here you can access your mailbox and profile page. Going forward I’m going to write up some articles on how to use some of the features on the site to get the most out of it.
Anything else let me know,