You need to visit your GP. They tend to either refer you to a psychiatrist or sometimes some areas have specific centres for testing adults with Autism.
If (this is the case for some adults) for example, you're being treated for depression or a related mental health issue via community mental health, you can ask them for advice. They may not be able to diagnose you; but might be able to refer you to the appropriate people.
When I was seeking a diagnosis, I wrote down why I thought I have Autism. One of my problems is that I can't explain too well what I mean. You also need to go into the appointment with your GP with seeking the diagnosis being the only issue you're asking about.