If you want to use a Debit Card--that card must have either a MasterCard or Visa log on it. Otherwise PayPal will not accept it.
If it DOES have the coerrect Logo on it--then you must go to your PayPal account--and register that Card with them. They will ask your for your Card #--and maybe some other info too.
Once you have done that--you will be able to use it to pay for your purchases.