First, do you have a medical diagnosis from a doctor? If not, find a developmental pediatrician or pediatric neurologist and get a full evaluation.
Second, find out what you qualify for. Contact SSI, your state's center for disabilities, your local school district and your insurance company.
Next you're going to need to put together a therapy schedule. ABA should be your first priority. Speech therapy and possibly occupational therapy will probably also be in there somewhere.
Expect waiting lists and don't go with whoever can get you most quickly. Everything will feel very urgent at this time but you're better off waiting some extra time to get good doctors and therapists.
Depending on your state, school district and insurance, this process may be relatively easy or very complex and time consuming but remember that this is the foundation for your child's life. It will feel overwhelming for a while but after a while you will settle into your new normal and you'll be ok.
One last thing. It's ok to grieve for the child you thought you would have. Let yourself feel those emotions so you can work through them and accept the wonderful child you that you didn't expect but will amaze you everyday.

The best thing we did was getting started on biomedical treatment. DD had been receiving traditional therapies (speech, OT, PT, an ABA-based Early Intervention preschool, etc.) for almost a year for general developmental delay and had been making "slow but steady" progress. But we saw dramatic improvements literally within days of adding certain biomedical treatments. I highly recommend reading "The Autism Book" by Dr. Bob Sears and "Healing the New Childhood Epidemics" by Dr. Kenneth Bock and finding a doctor familiar with treating autism biomedically.

First off try to take a breath and know that you are in the beginning of a long process that will be life changing. You have to accept the fact that your family is not going to be the same as friends and family. Others not in the same situation are not going to understand what you are going through. They may mean well with their advice but often it is going to not be inline with your reality. You will soon find out who your real family and friends are. You may find you are not invited to attend events everyone else goes to because people do not want to be inconvenienced by your child. I am not being negative just stating the real truth.

Do your research and prepare for long waits for appointments and therapies. Know your child's rights because your state and school districts will not offer anything. They do not want to have to spend money unless forced to sometimes. When asked to get evaluations done you should do it even though it is most likely just a ploy to put you off and delay services because you will ultimate dues it against them. Documentation is everything so get what you can even if it is frustrating . Finally if you are in a state with poor services consider moving to somewhere better. You have to fight for your child so prepare to spend a lot of time advocating for your child.

Apply for SSI and try to get services like speech, OT, PT and ABA they will help.

You can see if you qualify for SSI. We waited a while before we did it because my husband was scared of the stigma. However, if you do qualify you get lots of help with therapy and medical. It will help pay for diapers if your child is not potty trained or for equipment that helps your child if your child's therapist writes a letter. Having a social worker that is familiar with everything in your area is helpful.

@kathrinadenning the terminology can get a bit confusing, especially because not everyone uses the terms the same way.
I think the answer to your question is that a medical diagnosis would come from a doctor - ideally from a doctor who specializes in developmental delays.
There are others like professionals at your school system, for instance, that can suggest your child probably has autism but that's not a medical diagnosis and should be confirmed with a doctor.
Our school system never went further than saying my children showed red flag behaviors and honestly I think it's overstepping their bounds for someone who's not a doctor to say that your child has autism for sure but I know it happens frequently.