Ca159 is correct. I was a certified diver before going to high school. You start with open water training because you can always go up to the surface at a controlled speed. Then you can get involved in ship wreck driving and cave diving if you choose to continue the training. I was trained at a local YMCA. Classes are not difficult to find and it only took less then two months of classes and real diving. Fun stuff, good luck.
All you have to do is take a certification course which might be 4-6 weeks long and includes classroom time and also time in the water. They also have more advanced courses if your interested in taking things further.