Depending on the parent of course, it can go either way. There's no right or correct way to go about this but the idea is to be honest. Sometimes you have to sort of give vague details as not to worry some of the more conservative parents.
Get to the point, be respectful, and be ready to answer any questions that are shot your way.
Be as truthful as possible, of course you might have to stray from complete honesty but you want to do it in a manner that won't haunt you.
Just keep your confidence, whatever happens happens. Keep in mind the relief you'll feel after getting it out and you'll do fine.