Private messeages

How do i send a private message to 1 player without everybody being able to read it?

You click on their name, no one will read it.

In the game, You can either click on their names or wrote "/pr PlayerName" [without the ""].
If you want's to send Private Message via website then you got to go to the player profile you want to send a PM. Below his/her avatar there is a little icon of envelope in the left, You click on it and wrote the thing's you want.

Skull is also correct.