Or you can share your computer's printer with an app on the computer itself. OPrint for Windows or Printopia for OS X have extras. handyPrint for OS X is free, and basic. I've been using Printopia for years, and have never felt the need to replace my printer. (though when it breaks I'll almost certainly get an AirPrint capable one).
The computer must be on for this method to work.
The advantage of using the computer based apps vs apps on the iPad is that all AirPrint enabled apps will work normally, as if you had an AirPrint capable printer. The iPad based apps have their perks too, especially if you get one specifically for your printer/brand.
You should take a close look at both before deciding.
There are other options, but the two mentioned so far are the most common, and easiest to implement.