Your iPhone already comes with everything you need to capture amazing videos. For better audio quality, you should consider investing in a decent external microphone. The MicW iShotgun II Microphone for iPhone is worth a look. It is a short shotgun microphone with a Lightning connector. It has a supercardioid pickup pattern, making it ideal for interviews, speech, and music.
This kit also includes a cold shoe mount, Lightning extension cable, and windjammer, just in case you want to record audio outside. More info is available here.