How do I use debit cards on internet payment systems?

I need to know how to use debit cards on paypal and stuff like that. Mostly it asks you for credit card numbers and a security code at the back of them. But how do you just use a debit card number?|||Most sites consider a debit card the same as a credit card because you're paying electronically through the card. Generally, if it needs to know if it's a debit versus a credit card, it will either ask you to check a box or fill in a specific box for the credit or debit number. |||the same way as the credit card.