Why can't I download free apps anymore? Since ive purchased 1 app using a credit card. Now every free app brings me to the payment screen????

You are required to provide credit card details to download apps be it free or paid.

Nothing will be deducted from your credit card if the app is free.

There will be a authorizing hold of US$1.00 to verify your account information. It will be credited back to your account later.

After establishing the Credit Card account (i.e. after you have made purchases), you can remove the credit card information by editing your payment info, choosing "None".

Settings>iTunes and App Stores>Apple ID>View Apple ID>Sign-in>Payment Information

Accepted form of payment

http://support.apple.com/kb/HT5552