Q: Air printing from an IPhone to HP Printer, how do I go about it?

iPhones and iPads can print directly to an AirPrint compatible printer.

Check to see which model of HP printer you have and compare it against the list of compatible AirPrint models in this Apple support document:

http://support.apple.com/kb/ht4356

If your printer is on the compatible list, and you are still having problems, you may need to download and install an update for your Printer. Contact HP support in that regard.

yes you ideally need an airprint enabled printer

if not, try an app like hp eprint or print n share:

http://mobile.eurosmartz.com/products/printnshare.html