What should I do if I see a pressure sore beginning?

Move. Pressure sores HAV many contributing factors, the one common cause is pressure."off loading" is necesary for all preventive regimens.
Pressure relief. The best thing to do for an early pressure sore is to stay off it and keep it clean and moisturizer. If you think it is more advanced seek medical care.
See your Doctor. The primary cause of pressure sores is infrequent changes in position. Chronic illness and poor nutrition can also contribute. Prevention is of course the best strategy. If you see signs on the surface of the skin, it can sometimes be masking a deeper problem or even an infection. This should be evaluated by a physician so that treatment can be started if needed.