To be able to take yourself to the doctor without your parents knowing about it you have to be at least 15 years old.
Once you reach age 15 you can set up your own doctors appointments and go to the doctor without your parents being present.
You can also see a doctor at any age without your parents being in the doctors office exam room with you as long as your parents set up the doctors appointment and approves of you being in the doctors exam room on your own.
My 7 year old son does not want me in the exam room with him at his pediatrician and so I let him in the doctors exam room by himself.
He does good and I trust the doctor and so I wait out in the waiting room for him.
Then the doctor just lets me know the results of his checkup and tells me if there's anything wrong.
Once you get to 17 and 18 year you can take care of your own doctors appointments and do not have to tell your parents about the exam results.