I have ADHD, here are some things I would keep in mind:

ADHD is usually present with other neurological conditions: Tourette’s, OCD, ASD, dyslexia, audio-processing issues. Research these disorders!!

What kind of ADHD is your character going to have? Hyperactive-impulsive, inattentive, or combined? HI is most commonly diagnosed in boys, and inattentive is most commonly diagnosed in girls (but either gender can have any type). HI = impulsive, fidgets and talks a lot, can’t sit still for long, restless, no filter, speaks at inappropriate times, grabs things from people without asking first, hard to wait their turn, hard to listen to directions. Inattentive = difficulty with organizing and finishing tasks, daydreams a lot, trouble with paying attention to details, forgetful, easily distracted, hard to follow instructions and conversations. Combined = ~equal amount of symptoms from both lists. Severe ADHD doesn’t always mean hyper, it can be severe no matter what type.

It’s common to have trouble controlling their temper, which can create issues with socializing.

Give your character a hyperfixation, they can change it throughout the story or keep it. It can be a certain topic, song, book, movie/tv show, hobby, etc. (For me, personally, as soon as I’m done hyperfixating on something I’m completely sick of it and try to avoid hearing/learning about it for weeks. I move on to the next hyperfixation and ignore my previous one.)

People with ADHD can fidget a LOT. Bouncing their leg, tapping their fingers, playing with whatever they are holding, rocking back and forth, etc.

ADHD meds aren’t the holy grail. They don’t always work for everyone, and stimulants can cause tics. If you are going to medicate your character, research the difference between stimulants and non-stimulants, they have different side effects and work pretty differently. Caffeine can be a useful alternative to medication, it helps with focusing and energy (unlike people without ADHD, I can drink coffee at 11pm and go to bed right after. It actually makes me tired, not energetic, which calms me down and lessens fidgeting).

Therapy can help with ADHD symptoms, a combination of therapy and meds would probably be best for someone with severe ADHD.

Boys are more likely to be diagnosed, and get diagnosed younger. However, if they have inattentive type than they are usually called lazy or absent-minded. If your character is a woman (or a man with inattentive type), realistically they are more likely to be ignored by the school system and will have trouble receiving help/a diagnosis.

Good places to learn about ADHD from people with the disorder are How To ADHD on YouTube and adhd_alien on Instagram/Tumblr. Both of these women make content about ADHD, very helpful!!