Simply click the Drink Bar icon at the left of the screen and you'll be allowed to place it in your cafe. After doing so, you will need to build it first, and for that you need help from your friends. Here are the material requirements for completing the Drink Bar in Cafe World:

 - 8 bucket of ice
 - 8 menus
 - 8 napkins
 - 16 kitchen tool

Once completed you will be able to serve drinks to your Cafe World cafe customers, probably in a similar matter to that of serving coffee. However, since the drink bar is only a very new feature and it wasn't fully launched (the drinks are not yet in the game), we'll have to wait a tad more to get those details!