why can't i get junk folder or any other sub folder on my iphone. my daughter can and she has an iphone

how would we know? We're not in front of your phone to look at how it's configured or what folders you have setup to view. Look at her phone and compare all the settings.

Set up your e-mail account as IMAP, not as POP.

Chances are your daughter has an IMAP email account and you have POP. Only the IMAP type accounts allow creation of folders in the Mail app.