If you have an iPhone or iPad and an AirPrint capable printer, you can easily print from any app using the Share feature. You need to be on the same wireless network to be able to do this, and the printer needs to be switched on.
The problem comes if your printer doesn’t support AirPrint. Most printer manufacturers have an app you can download that acts as a helper so you run that app to do the printing.
If you have an older printer, such as one with only USB, then you can use software on a computer in order to handle the printing process. One example of this is Readdle’s Printer Pro. This comprises of an app and some software that enables the device to print via the computer.
Alternatively, for the Mac there is Printopia, which adds AirPrint capabilities to non-AirPrint capable printers.
For Android devices, you have to search for Printing in Settings. The easiest method of doing this is swiping down from the top of the screen and clicking on the cog. This will give a search symbol to which you can search for Printing.
Google provides Cloud Printing, and if the printer is compatible with Google Cloud Print, then you can add it to Cloud Print by clicking on the three dots and then choosing Add Printer. Some printer require additional plugins and apps to be installed in order to add the required printer.
To print from an Android device, click on the three dots and you will either see a Print option, or a Share and Export option which will lead you to Print.