How to take passport photo on my iPhone?

Before you do that, I suggest you consult the instructions for your country's passport photos. Not every country has the same requirements.

If you're in the U.S., start here:

https://travel.state.gov/content/travel/en/passports/how-apply/photos.html

Honestly, though, if you can, get it taken properly at a photo place. I had my most recent one done at AAA. It wasn't terribly expensive. And it saved me the trouble of having my renewal rejected because of an improper photo.