How to speak to a therapist?

In middle school my teachers told my parents that I needed a therapist because of my clearly flawed mental health, but my parents strictly prohibited it. To this day, at the age of 18 I am not allowed to speak to a counselor or therapist or anything.

But I've really been wanting to at least try it a couple times or hopefully even get a diagnosis. I don't earn enough money to afford it, and I do online classes so I don't have access to a school counselor. What can I do without my parents finding out?