After selecting multiplayer, you will enter the lobby. This is where you can see all the multiplayer rooms. The rooms are divided in 2 categories; Real rooms, where you can play for Real credits and win money, but also Fun rooms, this is where you can practice first. In the Lobby you can see how many players are playing per room and how many tables are occupied. Click on a room to enter. You need an account to play for Real.
Creating an account is easy and can be done here .