seems strange to be talking about ios6 when ios7 is out. if you are not into the jailbreaking scene, and unlike others, i like ios7 on my phone, why not update it to ios7
for whatever reason you want to still maintain on ios6, try toggling general settings> restrictions > allow to download/install apps
re enter your apple id, since this is the only way to download and purchase apps.
check that your apple id is linked with a valid payment, ie credit card