How can I easily learn how to subtract money?

Okay so for a school thing we have been given the choice to work In our canteen for a day or so but whatever and with this you have to do the obvious stuff, take their order give them the food and give them their change as well. That's one thing I'm afraid I will be terrible at, giving them the right change quickly. So do you have anyways I can learn to subtract money in my head quickly?? Please help me- thankyou!