Your first stop should be your family doc. He should be able to tell you where to get evaluated.
In preparation for that I would strongly advise you to read up on the condition. That includes checking the DSM-IV entry for it and perhaps trying one of the online tests. But remember, none of that is more than indicative.
There is a huge number of issues and conditions that can mimic ADHD symptoms. Therefore, only qualified professionals can really diagnosed you. You need to know your stuff though, don't let them get away with half answers.