Only a few AirPrint capable printers will show up on the iPad without a little extra help.
iOS: AirPrint 101
Some other wireless printers (most so I've heard) will show up using the Print n Share and PrintCentral apps. These are an alternate way of printing that do not work directly with the built in Print (AirPrint) feature in most apps. For those few Wi-Fi printers that do not work, or wired printers, they have a companion program for your computer that will make the printers available. The comptuer must be on, of course.
If you have a main computer that is on most of the time you can use a third party program that will make your computer pretend to be an AirPrint compatible printer (and share it's printers). This works with the built in Print option in your apps.
Paid for, and worth it in my oppinion, are FingerPrint (Mac and Windows) and Printopia (Mac only). I like Printopia. It has a few extra features over FingerPrint.
Free is AirPrint Activator (Mac or Windows.) Very basic, only makes the printer show up.
The computer must be on, of course.