How do you get alzheimer's disease?

No one knows. There are some genetic compenents, and there are factors that increase risk. Those things that keep your heart healthy also generally help keep your brain healthy.
Cause unknown. It is a disease of aging, unless certain genetic patterns are present (rare). There seems to be a higher incidence with head trauma, down's syndrome, minimal exercise, lower levels of education, and association with diabetes, high cholesterol, high blood pressure. Some protection may be achieved by mediterranean diet, ingestion of red wine, cold water fish, coffee, etc.