As you're probably aware, Apple introduced its AirPrint technology with iOS 4.2.1. Under this version of the OS those apps that support in-app printing can print wirelessly to AirPrint compatible WiFi printers. Currently only a small number of printers support AirPrint - they are listed on the Apple web site. So, if your printer is one of those and you're using an app that supports in-app printing and you have iOS 4.2.1 then you'll be able to print.
Apps that support in-app printing include Apple's iWorks suite - Pages, Numbers and Keynote.
If your printer is not AirPrint compatible, then you'll need to use a third-party app, such as 'Print N Share' that cleverly inserts itself into the print menu of apps that support in-app printing and 'pretends' to be an AirPrint printer. If you choose the 'Print N Share' 'printer' when you try to print, the app 'translates' the AirPrint printing protocol into a form that your non-AirPrint printer 'understands'.
Print N Share supports direct-to-printer printing for most WiFi printers, though not all. For those that are not supported, it provides universal support via your PC or Mac - the company provides a small free app that sits on your PC or Mac and makes it appear like an attached printer so that the Print N Share app on your iPad can print.
By the way, if you choose the Print N Share route (I've used it with no problems) you need to be aware that under the multi-tasking operating systems iOS 4.2.1 Print N Share has to be 'running' in the background for you to be able to print. So, before starting an app, such as Pages, from which you might want to print, you'll need to start Print N Share and just leave it running.
Hope this helps - but don't hesitate to get back in contact if there are any further problems.
Tim
Scotland