How do you know if you have ADHD?

Hi im a 15 years old and i think i have ADHD. I've always had a hard time concentrating even since i've been young. I don't do that bad in school although it is mostly because i am naturally smart. But in the past few years i've been doing worse in school because i really just can not concentrate at all. I also have a hard time concentrating on other things not just school. I also procrastinate like crazy and even though i want to do the work i just don't. I am very forgetful and very disorganized. And I have a very short attention span. I also have extreme problems getting to sleep because i cant turn my brain off. Im not sure if i have it because i am not hyper all the time like most people with ADHD are but I do fidget a lot. Sorry that was kinda long but i was just wondering if i possibly have ADHD or not.