TKDTutor contains numerous topics that cover practically all aspects of Taekwondo, in specific, and the martial arts, in general. A great amount of information is contained in these topics. What do you really need to know? Below are most all the things you really need to know. Everything else is just superfluous mumbo jumbo (foolish or meaningless incantations; or ritualistic or ceremonial nonsense).
A person with good athletic skills can be an effective fighter in any martial art. Some arts specialize in some aspect of the martial arts, such as kicking, throwing, grappling, etc., while other combine all or parts of other martial arts to form another martial art. While there is no one best art, there are plenty of arts that are rip-offs. Like in any other decision you make, do a lot of research into the subject.
Pick a martial art that best suits your needs. Pick a legitimate instructor that you like and who has a reputation as a good instructor and who does not rip you off or belong to an organization that will rip you off.
Learn all you can and critically analyze all you see and hear.
Work hard and train as much in class as you can. Train at home as much as you can. Cross train in other sports when you can.
Learn the basics, practice the basics, and concentrate on using the basics in every technique or pattern or when sparring.
Respect you instructor as you would any other teacher, but do not worship your instructor. There is nothing special about martial art instructors that makes them any more important than another other professional teacher. If your instructor thinks or behaves as a little god or goddess, find another one.
Most martial arts have an oriental background so they are steeped in Far Eastern culture. Westerners have a curiosity about Eastern ways and thinks they are interesting. Different martial arts use varying amounts of Eastern culture, but none is required.
Simple works best. When your instructor tries to make something simple into something complicated, secret, or mystical, find another instructor. This is not rocket science.
Ki, chi, internal force, in the zone, etc
These are basically all mumbo jumbo. All they really involve is concentrating your mind and body on the task at hand with such intensity that everything else becomes secondary. Many athletes of other sports speak of this feeling, they just do not make it into something mystical and complicated. Any athlete that is a the top of his or her game experiences it.
Breathe deep, slow breaths through the nose using the diaphragm and exhale slowly through the mouth using the diaphragm. That is about it for breathing. Everything else is mumbo jumbo.
Get into a relaxed position in a quiet place, close your eyes, and breath as described above. On the exhale, use to your breath to faintly hum some meaningless word (such as "Ohmmm"), relax the body, empty your mind, and concentrate on the meaningless word. Do this for 10 to 20 minutes at a time at least once a day. That is it. No need for transcendental meditation, yoga, or any other meditation mumbo jumbo.
Toughening the hands or any other body part is unnecessary unless you make your living wrecking buildings with your body.
There are hundreds of self-defense techniques used by numerous styles of martial arts, but self-defense is really very simple: Block'em! and Sock'em!