Bad blower motor resistor. It usually is bolted next to blower motor on passenger side under dash.
Looks like this.
Your fan switch button is the one that controls how fast it runs from slow to fast. The button you use to turn on is faulty. Ita an easy fix just make sure you remember how to re-install the parts you had to remove it.
Looking for a Used F-150 in your area?
CarGurus has 207,570 nationwide F-150 listings starting at $1,273.
Search Ford F-150 Questions
Ford F-150 Experts