How to connect my iPhone to a printer

If your Printer is Air Print compatible, you don't need to do anything. The Printer will just appear in the Print option of any app, that supports printing in the Select Printer screen when the iPhone is connected to the same Wi-Fi network as the printer.

See here for details: Use AirPrint to print from your iPhone, iPad, or iPod touch - Apple Support

And here for AirPrint compatible Printers: About AirPrint - Apple Support

Do you have an AirPrint printer? Start here: https://support.apple.com/en-us/HT201311