There are a number of ways to print from the iPad. The easiest method is to own or purchase an ePrint cloud-connected printer from HP that works with Apple's AirPrint technology to enable wireless printing. If you have an older printer with some miles left on it, however, there's no need to rush out and buy an HP ePrint printer. There's an easy, if limited, way to print documents from an iPad using Google Docs.
For older USB printers without networking capability, connect the printer to a PC or Mac. Install Google Chrome on that PC or Mac. In Chrome, go to Options (Preferences on a Mac) and click the Under the Hood menu option. Scroll to the bottom of the Under the Hood page and click the button labeled, "Sign in to Google Cloud Print."
Once that's done, click the Manage Print Settings... button, and on the Printers tab you should see your printer listed.
Then, using Google Docs on your iPad, choose a doc you'd like to print, tap the double down arrow button on the right (it's located next to the Edit button), and select Print. You'll be taken to a page where your printer should be listed. Tap to highlight your printer, and then tap the Print button in the lower-right corner of the screen.
If you don't see your printer listed, make sure you are using the same Google account as you did to sign in to Google Cloud Print on your PC or Mac. Also, this method lets you print only docs or spreadsheets created in Google Docs; you can't print Word docs or Excel sheets previously uploaded to Google Docs. And you can't use this method to print photos from the camera roll or pages in Safari.