I cancelled payment method For icloud. How do I add a new credit card?

If it was me, I would keep it.

If you are using 8GB of storage, and you want to keep all of that iCloud content, yes you should keep the 50GB plan. If you downgrade, you will lose all of the storage over the free 5GB of storage that Apple gives you. In addition to that, there is no in between plan from the free 5GB of storage to the 50GB plan.

If you do want to downgrade, take a look at this and see if there is anything that you wouldn’t mind giving up so that you can get down to 5GB or less. Remember though, that if you are that close to being full in iCloud, you won’t be able to backup anymore.

Manage your iCloud storage - Apple Support

See this for information on how to add a new credit card to your payment options.

Change, add, or remove Apple ID payment methods - Apple Support

No. You don’t have to sign up for iCloud again.