I have a constant ache in my heel. Most painful when I first stand or walk. What can this be?

To find the cause... Heel pain can result from a number of different reasons. I would seek the attention of an orthopedic foot and ankle surgeon. One in your area can be found at www.Aofas.Org.
Plantar Fasciitis. It could be plantar fasciitis, which is inflammation of the tissue on the bottom of your foot (plantar fascia) this tissue connects the heal to the bones in the toes and creates the arch of the foot. Symptoms can include pain and stiffness in the heal especially painful during your first steps in the morning. Another possibility would be a heal spur. There are good treatment options, see your dr.