One way to join a clinical trial is to be invited. If a doctor thinks you may meet the criteria to participate in the study, then they may tell you about the trial to see if you want to learn more about the trial and what is involved with participating.
However, you do not need to wait to be invited! If you learn about a clinical trial that is of interest to you, the first step would be to discuss it with your regular neurologist, who knows you the best. Your doctor can find out if you are eligible to join the trial. If you are, then your doctor should be able to get you in touch with the people who are running the trial.
You can also contact the company who is responsible for a trial (study sponsor) that you are interested in learning more about. The company can help get you in touch with a clinical trial location nearest to you to discuss more details.