First you need to be logged into the site. Once you are logged in you have a number of different ways to speak to people:
You probably know the person you want to contact because they have answered a question or answered something you are interested in, click their name and that will bring up their user profile. Near the top you will find a "send private message" link, click that and follow the instructions.
If you have already traded some messages before you can hit "My Updates --> My Messages" which shows the last message sent/received from each of your friends
Alternatively hit the envelope icon at the top right under the "logout" button, that will bring you to the "My Messages" page.