Give yourself time to process this, it has obviously come as a huge shock. Treat it like any other situation in life where you feel the rug has been pulled out from under your feet. It will be okay, it's not a dire diagnosis, it's very helpful to know so early so that the right interventions can be put into place.
When you get the written reports, it will seem even worse because they only concentrate on the areas where your child needs help, so it will seem even more overwhelming and terribly negative. remind yourself it does not give the whole picture of your boy.
Keep remembering your lovely little boy is the same little boy he was yesterday, and he will be your lovely little boy tomorrow, the diagnosis has not changed him, it's only there to get him the help he needs.
This is a good explanation to start you off.
the-art-of-autism.com/understanding-the-spectrum-a-comic-strip-explanation/
It may be advisable for you to fully understand his diagnosis before telling family and friends as some can be far from supportive, usually based on not knowing anything about ASD but having a negative opinion anyway.
In time, you can look and see if any other family members are neurodiverse. It's common for at least one parent to be, and they often don't see the signs because the autistic behaviour is just ordinary to them.
For now, just take each day as it comes and allow yourself to process all the new information. 