Thinking that you know everything there is to know about diving is not a good thing. I have been diving for a long time now 15+ years and I still learn something new every dive.
My other piece of advise would be that your LDS (Local Dive Store) can be your biggest supporter. Hang out in there, spend time getting to know the people who work there, get to know the divers who come in, just talk diving. Those who support their LDS tend to get the better deals on equipment, the better dive maps, the better advice, and sometimes even free stuff.
I would also suggest talking to the instructors before taking the class. Find an instructor with a style that you will do well with. They are people with personalities just like everyone else. If you aren't going to be able to learn from them, take a different class with a different instructor. I wish someone would have told me I had this option. Most places have more than one instructor. You may have to wait for the next class to start, but it is worth it. You OW instructor may just become one of those people who is there for you when you need someone.
Get the training, take more formal training, and always be aware that a mistake can be fatal. Diving is fun. Dive with the respect for the ocean/body of water that it deserves.
Tim