Apple's own support pages on AirPrint has documentation on what kind of printers support the protocol. Several of the larger manufacturers, such as HP, Brother, Canon and Epson all have models that support it. The printer you already have may support it, but it might require a firmware update. Also, to use Apple's own words,
"AirPrint printers connected using Bluetooth or shared through a USB port of a Mac, PC, AirPort Base Station, or Time Capsule are not supported."
Once you've determined that your printer and setup is AirPrint compatible (like my WiFi-connected HP LaserJet 100 shown above) and it is connected over the same Wi-Fi network that is accessible from your iOS device, you then have to actually use an application that supports AirPrint on your iOS device. Not all iOS applications support AirPrint.
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