How much do you want to spend??
is it presentation style, action? Loads of walking and talking or quite static?
you can get Decent lav (personal of tie clip is another name) for an iPhone that will work but you are restricted to the length of the cable. Of your doing loads of action/movement you might want to think about a radio mic set up, rode is a fairly decent make but that is getting you Possibly into new camera territory. You can get cages/rigs for the phone that you can then attach accessories to.
the phone is cheap and in fairness could work well. You need to identify exactly what you want to achieve which will then inform what kit you need.. how are you editing the footage or is it a one take wonder???
feel free to DM if I can help more!!!