There are a number of variances within ADHD and other medical conditions that share symptoms. It is extremely important to know exactly what you're dealing with before you seek treatment. It's like treating the common cold with antibiotics - they don't have any effect and the cold won't go away, likewise you'll be putting things in your body that you simply don't need. However, with ADHD an inaccurate diagnosis and mistreatment has much more significant risks because ADHD doesn't go away like the common cold. It affects every aspect of your life.
So your absolute first step is to ensure a proper assessment and diagnosis.
Where do you go for your assessment and diagnosis?
Ideally you want to get a proper assessment and diagnosis performed by an expert in ADD/ADHD. For children and teens with ADD this is often easier simply because there are more pediatricians and child psychiatrists comfortable with ADD than there are adult psychiatrists comfortable with adult ADD.
If you're an adult looking for a specialist who is an expert in ADD, then the best place to start is with your family doctor, or primary care physician. The next step (if your doctor doesn't know who to send you to) is to contact your local medical school, or the biggest medical school close to where you live. Speak to the secretary of the department of psychiatry and ask about who is working with adult ADD.
I also recommend asking around your community. You might be surprised to find out who you already know that has dealt with ADHD, they may know of some local experts.
What to expect from the assessment and diagnosis:
Whoever you go to, you want to make sure a thorough history and a physical examination is conducted. Generally speaking you will want to spend at least 45 minutes to an hour on the thorough history. Some doctors may break this up into several appointments, but just know that diagnosing ADHD in about five minutes is generally poor medical care. The doctor will likely give you some questionnaires, ask about background information, and schedule another visit.
The physical examination itself is very important. If you're seeing a psychiatrist for an ADHD diagnosis, it is also important to see a physician for the physical exam portion. Psychiatrists generally do not do the physical exams.
The physical exam is important because it helps to rule out other conditions. For example, if somebody has a hearing problem, and they are not paying attention - it's because they can't hear properly. So the proper treatment is a hearing aid, not an ADHD medicine. Right? There are many background conditions that can cause people to believe there is an ADHD condition when there really isn't.
To begin treating ADHD without a proper diagnosis simply results in treating the wrong thing. The problem doesn't get resolved, in fact things can become worse, and you're left with a medicine cabinet full of medications for improper diagnosis. Take the time to find a qualified ADHD specialist and obtain a proper diagnosis and assessment. It will mean all the difference with your treatment experience and success.