My blood test shows high lipids? when should medicines be considered, since some people are just on "diet"?

Depends: Depends on your risk factors, whether you have documented coronary artery disease. If you are diabetic you probably should be on a statin. Diet is astaple of treatment but may not be enough. Your treatment plan needs to be tailored specifically to you. See your doctor.

On diet: On diet or change eating habit is the major and first step to improve the high lipid. Indeed, most of people can improve their lipid profile by diet control. If that does not work or does not reach to the gold, adding medication will be the next step.