I don't know much about Catholocism, so I can't say much from experience. Is it similar to Baptism? If you are an adult, how you tell them should be up to you. If you're still a child, I would suggest doing what the Bible tells us to do by honoring your parents. If they want you to do this, I would let them know, respectfully that you aren't comfortable doing it at this time.

If you think that you're not ready for confirmation, then tell your parents and your instructor. I know this is acceptable, but I couldn't tell you why.
Also pray to God about it.
Maybe tell your parents "I think god is telling me to wait a minute before being confirmed" or something.