If you only want the free iPhone apps, take a look here:
http://support.apple.com/kb/HT2534
Read the steps carefully as the order in which you follow them is apparently critical. This seems to come and go, or only apply to App Stores in specific countries, so you may not see the option for "None" when asked for a payment type.
Note that this only applies to the App Store; you will not be able to get any of the free content in the iTunes Store without entering in some sort of payment method (credit card, prepaid iTunes card, gift certificate, etc.)
Regards.