First step is speaking to your parents about how your feeling (if your comfortable with it, obviously it is very nervracking but hopefully it will come out okay)
If you are over 16 you can speak to your GP without any parents needing to know
However during the assessment they speak to an informant so it's advised that you take a parent or someone who has known you since birth
Good Luck
Also the ADHD and ASD assessment are done at the same time due to the high cross over so being assessed for one will do the other (not all private do it together)