WARNING: It is forbidden to talk about sex in public rooms. You will be banned if you talk about sexual matters in public.
The public chat rooms can be accessed by using the menu.
When you arrive in the chat lobby, you can join one of the opened chat rooms.
You can also create your own chat room and people will come and talk with you. You need to give a name to the chat room when you create it.
Instructions on how to use the chat panel are here.