The first thing you do is find out as much as you can about it and then go to your GP and ask for a referral for adult ASD assessment. He/She will ask you what makes you think you have it, and you need to be prepared with the answers. Write it all down and take it with you. Everything you can think of and go back as far as you can, including things you remember from being a child etc. If you're very lucky you'll get a GP who knows something about it.
It varies depending on where you live, but in my area I got referred to a place where I had about an hour's appointment where I had to answer lots of questionnaires and was told I would be put forward for the full assessment and was warned of a long wait.
The waiting list turned out to be 3 years long and the assessment with the clinical psychologist lasted 3 hours without a break (I've heard it varies). If possible they will want to talk to your parents too. Was impossible in my case, so they spoke to my husband instead. I took pages of things I thought were relevant, and old school reports.
I got asked a lot of questions. But they aren't just listening to your answers and writing it down, they're also observing your body language, posture, gestures etc. I had to look through a picture book and make up a story to go with it. Same with the random objects she put on the table and asked me to make up a story about them. Had to mime brushing my teeth. Looked at a 'Where's Wally' type picture and was asked questions about it. She literally wanted to know everything about my entire life. I had 2 meltdowns in those 3 hours. After waiting so long I was extremely stressed out about it. Sitting in a small room with a stranger who I knew was trying to find the real me inside was traumatic. Not everyone has that experience, of course.
Then it took her 7 months to type up a report and diagnosis and send it to me. To be fair, she did only work 2 days a week, but my husband had to do an awful lot of chasing up.
She told me at the start of the assessment that I wouldn't get an Aspergers diagnosis, that if I got diagnosed at all it would be ASD, because Aspergers is included in that diagnosis now.
In my case, when I first read about it and thought "that sounds like me", my husband also said he thought I had it. Husbands/partners spend a lot of time with us and see behaviours that we don't realise we have, I think.
Is it worth it? Yes. Absolutely! There were people who were very skeptical that I could have ASD until I got diagnosed. I feel validated. It feels good to know what I am. That all the things I thought were 'wrong' with me aren't wrong, just different. To have answers as to why this happened or that happened. Everything makes sense now and I can go a lot easier on myself. I have changed because now I ask for help/accommodations when I need them.
Is there a treatment that helps? Not that I know of. I got my diagnosis and never heard from them again. There's very little outside support for adults. There's a wonderful community online though! PM me if you want links.
I hope my post hasn't put you off. My area is notoriously bad for this sort of thing on the NHS. You might be able to go private, or your area might be better. I live 8 miles away from the next county and I know people there who got referred, assessed and diagnosed on the NHS all within a few months!
Sorry this is so long, hope it helps.