How to use debit card to buy things online?

I'm trying to buy some books online right now but I can't because they only have credit card as an option. I don't have a credit card and don't plan on getting one, so could I still use my debit card? Do I have to register it on Paypal (I don't want to because I'm not even sure how that works)? Is there any other way? Also someone said I could use my debit card just like a credit card if it says Mastercard of Visa on it - i.e. just enter the details like I would with a credit card. Does that work?