How does someone get heart disease?

Depends. That would depend on what type of heart disease you are talking about. There are things you are born with, there are infections, trauma, and things that are related to risk factors. Coronary artery disease for instance is more common if you have diabetes, smoke, have a high cholesterol, and a family history of the problem.
Cholesterol deposits. Coronary artery disease is due to cholesterol deposition in the arteries. Risk factors that increase the likelihood of coronary disease include hypertension, diabetes, family history of coronary disease especially at an early age, high bad cholesterol (ldl) , low good cholesterol (hdl), smoking, obesity, lack of exercise as well as some inflammatory diseases.