How do you go to a therapist without your parents knowing?

In every country there are institutions that help people. Some oriented to mental health, addictions, etc and they are normally for free. Look one up in your country/region, make contact with them and for sure they can give you a first appointment where you can discuss with them why you do not want your parents to know. They will help you to solve the problem, either by keeping it confidential or by helping you in the right moment, find a way to tell your parents. But when you approach them what is important is that YOU have an issue that want to solve and you are seeking help. That is the valuable thing, don't waste it! Success!