Is a blood pressure reading of 143/84 unhealthy? Should I start watching my weight/what I eat?

Yes. Your blood pressure is dangerously out of control for an 18 year old. You need to establish with a health care professional to determine what factors you can change or modify (weight and food are only part of it) to get your blood pressure under control as soon as possible. You risk kidney failure, heart disease, stroke and premature death if you don't start doing something about it now.