In the App Store, it doesn't ask me to put in credit card info

Whenever I'm in the app store, and buy an application, it doesn't ask me for my credit card information each time - instead, it just goes through with the download (asks for password after a while).

Is there any way I can change the settings so that for each app purchase, it requires you to put in the credit card information? That way people can't randomly buy apps when I'm not on my iPad?

I share my iPad with my brother, but I bought stuff with my credit card on my iPad. He's been buying stuff with my credit card on the iPad - Does anyone know how I can force the App store to ask for credit card info for each purchase? I know that's what the password is for, but I share the iPad with him, although the credit card info that the iPad remembers is mine...

A little help?