Hello
Flat foot is foot with less developed longitudinal arches.
It can be congenital ( by birth) or acquired due to lifestyle activities and other causes also .
Next Steps
I suggest foot exercises like dorsiflexion ,plantarflexion , inversion and eversion twice a day .no. of repetition would be 10 and then further progress to more.
Only after proper examination of the foot ,we can prescribe foot arch support.