How do I print from my iPad 2 to my Lexmark 900 series printer?

Since it isn't a printer that supports Airprint (See here: http://support.apple.com/kb/ht4356) you will have to use one of the printing Apps available in the App Store. For example, Printcentral, Printopia, etc.

As well as Jim's suggestions,, if you have a Mac and other IOS deviices then Handyprint is also a reasoanble alternative that allows wireless printing from any device connected wirelessly to your wifi network.

Cheers

The Lexmark Pro900 Series is not AirPrint compatible but you can use the LexPrint app to print from an iOS device. You would also need to install the LexPrint Listener to a host computer which will serve as a bridge for your printer and iOS device. Just go to http://bit.ly/KY8eCI for more information.