@Crossbreed has a good idea,...having the "news" come from a professional may be more convincing. That said, a general practitioner probably isn't the best resource when it comes to the diagnosis of autism,...unless, of course, they have specialized training. I do hope that your process does involve a referral to an autism specialist in your area,...and let them perform the testing with you,...as you will receive much better information,...practical information YOU can use in your daily life. Having the information come from an autism specialist,...a much better idea, as your parents will surely have questions that only they can answer more appropriately. The more you know the earlier, the better off you will be. The earlier you understand yourself, the earlier you can "work around" your autism,...perhaps do things differently than others, but accomplish the same goals in life using a different pathway.
I have no idea of what kind of father you have in terms of empathy, education, love, etc. Some fathers you can just speak freely with and they will take your concerns to heart,...and on the other extreme, some father's don't want to hear anything about a "diagnosis",...as it may reflect badly on them. I know,...the later situation is twisted and horrible, but it happens. So, I am hoping you have a good relationship with your parents.
That said, I also agree with writing a letter. As a child,...especially if you are autistic,...it may be difficult to get the words out in a meaningful manner. Most people, in general, will want to interrupt your train of thought,...be disagreeable, try to minimize what you are feeling, etc. Write it down, let them read, and that way you don't have to deal with the frustration of someone interrupting your train of thought.
Take care, and welcome.
