Q: which app to use as airprint from ipad

If your printer is air print enabled you need nothing. Air print to air print enabled printers runs natively.

If your printer is just a wifi printer that's not air print enabled then you will need to dig deeper. Perhaps the printer manufacturer has an app or work arounds. Or there are third party printing apps that you can use.

This explains how to print:

Print from your iPhone, iPad, or iPod touch - Apple Support