Very Important things to do before you Reach 20

Originally Answered: What is one thing you wish you had known at 21?
Cannot possibly single one out ... but here are some tips / advice ...
These are not your typical answers...

For the love of your own life, Don't smoke. If you're already smoking.
Quit. The excuse that work stress made you do it, is not worth it.
Get exercise to be part of your daily routine.
Exercise your brain continuously: Keep it busy. Play poker or chess,
or solve crossword puzzles, or strategy games. Keep it buzzing.
Travel. Seriously. As much as you can. Especially to conferences.
Food/Beverages: Learn, experiment, try out, taste all different types
of foods. You eat 3 times a day if not more. Everyone on Earth follows
the same routine, get to know food and you can break the ice in so
many situations. Besides, its very tasty!
Cooking. Learn it. It's fun. You will get to enjoy food more and the
company that comes with it.
Learn about Finances. Even if it might be boring, or not applicable
immediately to you, learn about finances. To make money, you need to
understand money, unless you luck out (which very few people do).
Start saving. Get into the habit of tucking a few Dollars every week.
Try to build up savings for a rainy day.
Music: Appreciate it. Read up about it. Listen to as many different
genres as possible. Everyone loves music.
Invest in property. As early as you can. Property in the long run,
will always give you a profit.
Self-Pity - It will eat you up. Don't feed it. Cannot stress this enough.
Be Happy in whatever you do. If you're not, why push yourself through
it (be it life, work, personal relationships, hobbies, etc.)
Read. Read as much as you can, especially History and People. So many
things will make sense to you.
Trivia: As trivial as it may sound (no pun intended), trivia is a
great knowledge source and can come in so handy in life when doing
small talk or trying to break the ice.
Love the Outdoors. The more you are out and away from your desk, the
greater the chance of enjoying life and meeting up with people
(accidentally and/or by choice).
Face Your Problems: Don't delay. Paying attention to problems at the
earliest will prevent them from snowballing into something much more
bigger with disastrous results.
Aesthetics: Though this one is a tough one to master, try to learn
aesthetics, knowing how to differentiate between the good, the bad and
the ugly.
Dress/Attire. Learn to dress well. It pays back in multiples in life.
Frugality. Learn it early enough in life. You'll thank yourself later
on in life.
Bide your Time: Decisions and opportunities will come forth a plenty
in your lifetime. Think it through. For most cases, never make hasty
decisions. Give it some time and see what/how you feel towards making
that decision. Patience is indeed an art.
Management: Learn and read up as much as you can about being a
Manager, Management Techniques, etc. Even if they may not be
applicable to you right away.
NO: Learn to say No. The earlier on in life you master this trait, the
better of you will be. It may be a sour thing to do, but you will feel
better afterwards. Most people hesitate or are too courteous to say
No, and end up burdening themselves.
Last but not the least, Deliver on Your Word. For your word is worth
more than your weight in gold. For you would be known by this trait.