As Jade has mentioned, you must really know your sub. Punishment must be tailiored to each person. And if you are new at this, you may find that you need to take your time to refine your skills with any toys that you use.
Being a dom requires you to put a lot of thought into what you do. Both for pleasure and punishment. Do not let your sub goad you into a course of action.
Having your sub fill out a questionaire is a good starting point. Once you see what they desire and what they fear you will be able to come up with a plan.
And remember, punishment does not always have to involve pain. My slave is very claustrophobic and I often use this fear to punish her. She likes pain, so why would I spank her.
Maybe your sub needs you to humiliate him, no bathroom may work. Put him in a diaper. But until you know what his limits are and what he needs and wants a plan will be impossible to figure out.
More study will help you.