You need to press your avatar, then the button with an image of a chip while you are at the table. This allows you to either reduce the balance at the table or get the chips.
Before starting the game, you can choose the table that best suits you, according to your current balance or desired bet size. You will always start the game with a certain balance. If you want to change the current balance at a table after you enter a game, you need to press the "Stand Up" button, then rejoin the table after selecting your desired bet size.