How can you convince your dad to let you piece your nose?

Depends on why he's against it. Start my being extremely responsible. Then show him that you know how to take care of a piercing. Show him websites that you've looked at, articles etc. If he's afraid of you looking trashy or something, show him pictures of pretty people with their nose pierced. You can also show him pictures of the different types of jewelery esp. hider jewelry or retainers. It'd also be a good idea to take him to the piercing shop beforehand. The piercer could give your dad info on the piercing and your dad can see just how clean the shop is so he won't worry about infections/diseases. Good luck!