I started at BK on the 11th
Regarding the stressful workload, are you still in your training period? If so, ask the person doing the training if there is anyway you can improve your work procedures so you'll be more efficient and appear less stressed to your customers. Ask them during a break, away from the customers' ears.
If you are out of the training period, during one of your breaks sit and watch the person doing your job for you while on break and compare how they do things to how you do them. See if there is something they are doing more efficiently.
What is getting in your way when it comes to multi-tasking and being better organized? That's where you can start your improvement efforts. Detail the problems here for us and maybe we can help you figure out how to get better at it.