Is it safe to enter my credit card number for payment for storage on my Apple phone ?

I am confused. Are you asking two unrelated questions?

Credit card -- yes, if you have a secure passcode.

iCloud -- Click on this link to discover how to "Upgrade your iCloud storage plan". Click here--> https://support.apple.com/HT201318

Yes it is safe. Make sure your iOS is updated. There is on device security encryption.