But where to even start?

If you live somewhere with socialized healthcare, start by visiting your general practioner, your regular doctor or "doctor's house" and say you suspect you relate to having executive dysfunction (ED). Mind you, ED can be symptom of other things than just ADHD (like ASD), although that is the most common culprit. Hopefully they'll advice you on where to go next, in my case it was a psychiatrist where I had a waiting list of almost 2 years, but if you go private it's probably a lot faster.