How do I setup AirPrint to my hp j4680 office jet

There's a list of printers that support AirPrint, the native mechanism for printing from iOS, here:

http://support.apple.com/kb/HT4356

The 4680 is not listed, so unless HP has released an update for that printer recently, your printer does not support AirPrint. HP has an app that enables printing to some of their other wireless printers. Third-party printing apps such as PrintCentral are also available. All such apps have limitations, so check them out thoroughly before you pay for any such app.

You could also route the printing through your computer. If you have a Mac, Printopia might be worth a look:

http://www.ecamm.com/mac/printopia/

Fingerprint is another option that is available for both Mac OS X and Windows system. You could also consider an AirPrint-capable print server. Lantronix makes a couple of models, including one that supports a USB printer:

http://www.lantronix.com/it-management/xprintserver/xprintserver-t2.html

Regards.