Apps on your device typically have a print option somewhere. For example, in Mail press the reply button and you will see a "Print" option listed along with "Reply" and "Forward". In Safari, the share button provides a Print option. If you don't see these Print buttons, you may not have a compatible device. (The original iPhone 2G, iPhone 3G and 1st and 2nd generation iPod Touch devices do not have AirPrint capabilities.)
To read more about AirPrint, see Apple's About AirPrint support article.