First and foremost, it’s critically important that you maintain a regular sleep routine. Unplug your devices at least an hour before bed. No caffeine or alcohol at night. Go to bed and wake up roughly the same time every day. I recognize that having ADHD this might be extremely difficult to do so I recommend set alarms for yourself or even better, ask someone to help your enforce it.
Regular exercise also helps. Again, ask a buddy to help maintain an exercise regimen if you find it difficult to do it on your own. You will find that people are willing to help.
Try meditating regularly as well. Just sit straight up, close your, and just pay attention to your breath. It sounds trivial at first but the more you do it, the better you are at it. You can then apply this practice lying down when you are ready to sleep. I have found it has helped lull me to sleep as it clears out all the crazy thoughts racing in my mind or rather, I just don’t dwell on those thoughts for too long.