The first stage of therapy is assessment. This is a process where your therapist aims to understand the problems you want help with, and will ask about a broad range of things. Questionnaires are often a part of this too. As this process concludes, you and your therapist will come up with a label for your set of problems, e.g., depression, social anxiety, generalised anxiety etc. This is useful as it guides your therapist towards effective treatment interventions.
If you and your therapist believe that you have post-traumatic stress disorder, then the rest of this article will hopefully be relevant.
Please get started by reading through the information provided by the Royal College of Psychiatrists on PTSD (click here).