Login to iTunes Connect (itunesconnect.apple.com) and download the developer guide. There's a link at the bottom of the "Manage Your Applications" page.
Or, login to the iPhone Dev Center and take a look at the info on there.
If you Found Out how to do it could you publish my iphone app for me because im only 13 yrs old and my mum and dad wont pay for the developer thing it will only be a free app. Give me your email if you agree to to this my email is tyronepsp@yahoo.co.uk