How do I choose a ball before throw?

There are balls recommended for each room. To check them out tap the "Rules" button in the bottom right corner of the room card. You can't go wrong if you choose one of them!
If you don't have these balls in your Arsenal, visit the "Proshop" and buy the ball you want. Then go to your Arsenal and drag and drop the ball on the rack. Now you will be able to use it during the match. To change the ball during the game click on the rack icon in the lower right corner of the screen (for right-handed) or in the lower left corner (for lefties) and choose the ball you wish to play with.