Sorry to hear that your cholesterol is high, but congratulations on going to see your doctor
and having your annual check up and screening blood work done! This is an important part of staying healthy - as you can see, in your case you did not know that your cholesterol was high!
High cholesterol is caused by a combination of lifestyle factors and genetics. Lifestyle factors that contribute to high cholesterol include unhealthy eating, drinking alcohol, being overweight, and not exercising. Usually, focusing on those lifestyle factors will reduce the cholesterol to acceptable levels.
However, the other component that contributes to high cholesterol is genetics. Therefore, if you have a family history of high cholesterol, it is possible to have elevated levels even if you have an otherwise healthy lifestyle.
You will need to talk to your doctor about what needs to be done about this cholesterol. If the levels are very high, you may need to start right away on a medication to lower it. Most of the time,however, your doctor will focus with you on making some dietary changes (and losing weight if you are overweight, cutting down on alcohol, etc) and then rechecking your level to see if medications are needed.