A good place to start might be to look at support worker or health care assistant roles. I started out with a company called Choice Support working with adults with learning disabilities, then moved into a support worker role on a mental health ward in the NHS. I then moved onto working as a recovery worker in a substance misuse service, but my experience in support work really helped get me there. It would be helpful to look for support worker roles in the NHS, roles in mental health charities, substance misuse services, learning disability services, helplines, etc. Any helping roles would be beneficial!
You will also need to complete further training in counselling and/or psychotherapy alongside this work.