For direct printing from an app, that app must support AirPrint. If it does, there will be little doubt about it. You will find Print as an option in one of the menus, and you will be able to select and print to your printer.
Of course, just because the app supports AirPrint doesn't mean it will do a good job. That varies.
For apps that do not support AirPrint, you usually use the Open In option to send the file to another app, like Printer Pro, and then use that app to print.