Q: I want to purchase something that is free on on the App Store but it will not let me it says I need a credit card

You need an App Store account which requires a credit card to open even though the item is free.

You might try using an iTunes gift card to get by the credit card snag:

(From the FAQ link at the bottom of the App Store first page.)

Order of iTunes billing methods

When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.

The above is from iTunes Store: How iTunes Store purchases are billed

OT

The App Store account requires payment info. But you will not be charged for free apps.

Follow the instructions here to create an account without a credit card:

http://support.apple.com/kb/ht2534