Depending on ios of phone you shouldn't need a special app.
For example, when you are reading an email you can tap the arrow at the top( or bottom) and options appear. Tap print and as long as printer is on it should look for the signal and give you options. Just follow the prompts as you go along.
If the arrow does not give you options you will need an app to AirPrint.
It's simple on e you figure it out.