One of the first things you would want to do is to find out if the diagnosis of autism is correct. Research the symptoms of autism and other related developmental disorders. You may want a second opinion of the diagnosis.
Once you are sure it is autism you should read or listen to stories from other parents of children with autism. Find a website with a busy message board and read and ask questions. Parents who have been dealing with autism for years may have the best insight on what to expect and how you can best support your child. How your family will deal with day-to-day activities and home life will be of most importance.
Once you have educated yourself about autism the first person you would want to explain the disorder to is your child. At first your child may not understand the disorder. Before telling your child they may wonder if something is wrong with themselves, and wonder why people react the way they do when interacting with them. It is best to tell your child as soon as possible before they have experienced too many unexplained reactions from others. This is important as most people will not know the child has autism. Your child may develop low self-esteem thinking they are doing something wrong and have no idea it is because they are autistic. Having your child understand autism will most likely take some time to explain. There are many books written especially for children and help explain autism in a way that a child would best understand when learning about their own autism.
What about telling other family members?
Telling your family members such as Aunts, Grandparents, Cousins, etc. may result in many questions or personal judgments. Some family members could see your child’s behaviors as bad parenting and not a developmental difficulty. You will most likely have to educate other family members about autism. The way your child may behave should be explained clearly. For most people this will be their first real introduction to autism.