If you have an AirPrint enabled printer, it's easy - you simply use the Share button to print and any AirPrint printers will appear.
AirPrint 101: http://support.apple.com/kb/ht4356
If you do not have an AirPrint printer there are other options.
It will depend upon what make and model it is. If it is AirPrint enabled then it should just need to be connected to the same network as your iPad - the models that support AirPrint and how to use it is on this page. If your printer doesn't support AirPrint then you could see if the printer manufacturer has their own app in the app store that supports it, otherwise there are third-party print apps such as Print Central For iPad and Print n Share which might work with it.