I have a kindle, I need to set up default credit card to use for buying books please

no you did not buy it from this help forum .
This assumption is very ANNOYING !
suggest you read the information below on how to buy kindle books on line
http://www.wikihow.com/Buy-Books-for-Your-Kindle

set new credit card as default payment method

Usually means the system is busy. If this is constant try one of these contacts.
To contact us via phone:
http://www.amazon.com/clicktocall

To contact us via chat:
http://www.amazon.com/clicktochat

if you have an account with kindle where you can edit the details , go there and edit the credit card details
it is probably in the same area where you can edit your delivery details , pass word , phone number etc

What you really need to do is add your new Kindle device to your
existing Amazon Kindle account. The good news is if you bought it from Amazon new, Amazon automatically registers it to your existing account. So . . .

Go to the Amazon website using your laptop or desktop computer and log onto your Amazon account.

1)On the upper right of the page you should see *Hello (your name)*
Drop down this menu by hovering your mouse pointer on it.

2) Select *Manage your content and devices*

3) If you're not logged in, enter your Username and password.

4) If you're browser is set to save passwords an log in you won't
need to do step 3

5) Select the *Your Devices* tab

6) All your Kindle devices should be listed there, including the new
Kindle Fire.

7) You can click on your old Kindle and *deregister* it.

8) You can make your new Kindle Fire your *default device* and
you can edit your other Kindle devices such as any Android
Device onto which you have installed the Kindle App.

9) The default device status ensures new books purchased will
automatically download to that device when it's connected to
the internet via Wi-Fi or 3G depending on what your Kindle Fire
is configured with.

10) You can click on the *Settings* tab to manage your credit card and other information.

Contact Kindle support. Fire Kindle Echo Support

1. Go to Manage Your Content and Devices.
2. From Settings, select Edit Payment Method below Kindle Payment Settings.
3. Select the desired credit card information, or add a new card, and then click Continue.
4. Enter the desired billing address, and then click Continue. You will be taken back to the Kindle Payment Settings page where you can view your edited 1-Click payment method.

http://www.amazon.com/gp/help/customer/display.html?nodeId=201252570

contact Customer Support. They must be able to help you.

You need to change the number on the Amazon site. Go to Amazon, your account, Manage payment options, will probably ask you to verify log in info, Click on Add payment method, You MUST set new card as default. That may be what you missed. I did it the last time I changed mine. There should be an option to delete old numbers.

Have you been issued a new credit card? The expiry date on the site may have passed.