How do you order pizza online using a debit card?

Okay so I have barely any cash, but I have my new debit card with some money on it and I really want some pizza. How do I do it? I'm specifically talking about when it comes time to tip the delivery person. Is it like a restaurant where you have to put down an amount on the receipt, or do I just pay the normal amount and when he shows up give him whatever cash I have lying around? I'm confused.|||You pay the amount for the pizza and give cash (whatever you have lying around) to the delivery guy.