Lots: Family history, poor diet (high fat, high salt, low fiber, etc.), lack of exercise, overweight, obesity, sleep apnea, diabetes, carrying fat around the waist, overuse of alcohol, any smoking: all of these and more can cause high blood pressure (hypertension).
Many. Age, diet, obesity, lack of exercise, alcohol, sleep apnea, drugs, otc supplements, genetics, family history, stress, overuse of caffeine and other stimulants, prescription medications, maladies you were born with such as coarctation, pheochromocytoma, and many others. Staying healthy, physicall, and mentally and following a good diet, exercising and reducing stress help. Lay off the salt too!