Setup AirPrint Printer
1. Buy an AirPrint enabled printer; see list below
2. Run the CD provided and set printer and iPad on the same network
3. Start printing from your iPad using AirPrint
If your printer is air print enabled, you don't need an app. But if it's not, I use print n share. I also see Printopia and Print Central recommended.
Poke around the App Store, look for 4-5 starred apps and then read the specs to make sure they look good.
You may also want to look on your printer manufacturer's page and see if they have any app that can help you