I receive a free voucher from google worth $20(received it in peso, our currency) How can I use it if they need me to fill up a credit card and I don't have one.
Not sure about all the details that you have but if a credit card is a must you may be able to get a prepaid card.
Its would be better not to use this $20 voucher. Better buy a $100 coupon as you can use it only once. And if there is option of CC you should be able to use any card. Otherwise in few countries there is option to pay via net banking.