How to add a credit card to my account

On your computer's iTunes you should be able to edit your payment info by going into the Store > View Account menu option and logging into your account, and on your account's details page there should be a payment link. If you are doing it on an iOS device (iPad, iPhone or iPod Touch) then you could try tapping on your id in Settings > Store (Settings > iTunes & App Store on iOS 6+) and selecting 'View Apple ID' on the popup - that should also give you a payments link on your account's page.

its not letting me change it can i speak with someone

What do you mean by 'not letting me change it' ? What happens when you try to change your card ?

If you are getting a 'declined' message for your new card then for a card to have a chance of being accepted it needs to be registered to exactly the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you and your iTunes account are. If it is then you could check with the card issuer to see if it's them that are declining it (if you are trying to use a debit card then that appear no longer to be accepted in all countries, and pre-paid cards won't work).

If you want to contact iTunes Support (these are user-to-user forums) : http://www.apple.com/support/itunes/contact/