Given the mention of NHS on the sign, this is focused on the UK.

Step 1. Note down the various reasons you think you might have adhd. Also include times that it has actively had a negative effect.

Step 2. Make a GP appointment.

Step 3. At your appointment, explain that you would like a referral for a mental health assessment, since you believe you have adhd. The note previously can either act as a prompt for yourself or just give them to the doctor. They were mostly to crystalise your thoughts and stop you going blank at the appointment.

Congratulations, you are now on the (very slow) path to a diagnosis. There are methods to speed it up, but even if you just passively follow instructions, you should get there.

Different countries will have slightly different systems, but the broad approach should still work. For our American friends, you have our commiserations.