Your iPhone already comes with everything you need to capture great video. You are still going to need a decent microphone to capture better audio. The Comica CVM-SIG.V05 MI Lavalier Lapel Microphone can help. It is a clip-on microphone MFi-certified for iOS devices with a Lightning interface. It has real-time monitoring, so you can test whether things are working during recording.
This lapel mic comes with easy gain control wheel. It has a low-cut mode for recording in noisy environments. The battery lasts up to 40 hours. You can find out more here.