If you don't have/know your amazon account password, you would need that to proceed.
If you've been trying to access it without success, your account may be temporarily locked. You will need to wait a bit, I would suggest at least 24 hours, to allow the account to unlock, before you try to do anything else.
Then, on your computer, click HERE
Follow the prompts, entering your email address, and then you will need to go check that email account,
it will have a link that you will click, which will take you to a new web page.
It does not give you a password ***** use, instead it gives you the option to set a new password, which you have to enter twice, exactly the same both times, and has to meet certain complexity requirements. If that works, then you'll be into your account.
The rest to set up your card has to be done on a computer as well, on the amazon website. This process will not work from your kindle.
From your amazon account page on the computer,
First, go to Manage Payment Options:
there you can be sure your card/bank info is correct (right address, valid expiration date, etc)
From there, go to Manage Your Content and Devices:
On this page, click on the SETTINGS tab at the top right,
then you can from there set a default card to be used for one click purchases on the kindle.
Under Kindle Payment Settings, click on the Edit Payment Method button, and you can set the card you want used for your kindle purchases.
After that, reboot the kindle by pressing/sliding and holding the power button for 20+ seconds, and it should operate normally from there.