the creature can roll the die and add the number rolled to one ability check, attack roll, or saving throw it makes. The creature can wait until after it rolls the d20 before deciding to use the Bardic Inspiration die, but must decide before the DM says whether the roll succeeds or fails.
Your Bardic Inspiration die changes when you reach certain levels in this class. The die becomes a d8 at 5th level, a d10 at 10th level, and a d12 at 15th level.