You might feel most painful first thing in the morning because your plantar fasciatends to tight up overnight. (The area which is about 4cm forward from your heel) The pain often comes back to you after spending most of the day sitting at a desk. For some people, the pain decreases a couple of minutes after walking since walking stretches the fascia.
The most common cause of plantar fasciitis is because you put abnormal strain and stress on your plantar fascia on hard and flat surfaces - for instance, a longer and faster running, lots of walking.
"Inflammation of the thick tissue on the bottom of the foot (plantar fascia) causing heel pain. The plantar fascia (also called plantar aponeurosis) are bands of fibrous tissue extending to the toes." (NCBI)
Slight stretching or resting your foot may help to relieve the pain, but being on your feet for long periods of time or undue stretching of the sole of your foot may make the pain worse. Happily, in most cases, the pain will get better in time.
However, the following treatments may help to speed up recovery.
(Note: The best way when there is a feeling of pain in any parts of your body is that you should have a full discussion with your doctor about the pain.)