How do I lower my LDL cholesterol at 20?

Eat better. Ldl, your bad cholesterol is affected by genetics and diet. You can improve your diet by becoming more plant based since animal fat can increase LDL as well as sugar!. So try to get 5-7 veggies/fruits per day, eat lean protein only with no skin (grilled, baked or broiled), decrease dairy since that is also animal fat, and stop all processed and refined sugar. It can increase your LDL by 25%!
Depends how high . Lowering ldl-cholesterol ( ldl-c) can be accomplished in many different ways. Depending on level & cause would determine the methods suggested to lower it. If you have a genetic or inherited disorder that causes very high cholesterol since birth then medications may be required. If not a genetic or inherited cause then we start with diet, lifestyle, but every patient is different. See your doctor.
Find and Fix Causes. The first step is to determine why the LDL cholesterol is elevated at age 20. Your physician will need to check for several inherited and acquired medical conditions that can affect LDL levels. Another alternative is to see a lipidologist (physician specializing in lipid and lipoprotein disorders). You can find a lipidologist at www.Lipidboard.Org (click "verify status", then search your state).
↓ Sugar & Body Fat. Ancel keys promoted animal fat as causing cv disease in the 1950s based on theory & cherry-picked data. The more his ideas were adopted, the more obesity, diabetes mellitus & cv disease rates have dramatically increased. So be careful, see: nusi.Org for guidance. Also check actual lipoprotein values (not cholesterol): LDL always in nmol/l, best <700, HDL in µmol/l, best >10: far better indicators.