How do I propose a new Apple iphone application to Apple?

Apple does not really make apps, other than a few specific to Apple products - pages, numbers , etc

Like most companies, they do not accept unsolicited ideas.

Find an app developer and see if they are interested in your idea.

Start by reading this article:

Legal - Unsolicited Idea Submission Policy - Apple

You can submit feedback to Apple here:

http://www.apple.com/feedback

You can also ask in their forum Developer Forums

Do an internet search. Find a app in similar category as the one you're interested in creating and contact the developer of that app.