Don't worry, even though some hotels don't require a credit card, your booking is still guaranteed.
Once you booked your hotel room, simply present yourself at the hotel on the day of the booking. Provide, the confirmation email and/or your first and last name.
It is possible that the hotel asks for a credit card upon Check-in, this will be used as a guarantee.
As soon as you receive your confirmation, the booking number is a guarantee that your booking is firm and final.