Know your own price. Don't put yourself last. Don't be the lovable loser. There's nothing wrong with you, even with all your flaws. Your capacity to become something great is perfectly good, despite all your issues and critics and self-doubt. Plow on, it's a long road.
On critics: "It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat. " - Teddy Roosevelt
You'll have a LOT of critics and detractors - family first, close friends and acquaintances, rivals, etc. Do manage to see what's valid in what they're saying (whether method or substance). Know whether their criticism is coming from a good place. Throw away what's coming from the wrong place and sideline them as best you can.
Keep your enemies close and your friends closer. The switch may be quick.
Keep your ideals close to heart and don't blab about them at all. These will be beaten up harshly through your new adulthood. Whatever you manage to keep will be for life.
Move your ass! The status quo means you're going backwards, not staying in place.
***Don't wait until last minute to do things or to be effective. It creates an awful feedback look that's very hard to break. It only gets worse with time, if left unchecked. It breeds tardiness, lack of focus, procrastination, idleness, and other vices. HAVE A GOAL, ALWAYS BE ON A MISSION, BE ORGANIZED. BE PREPARED. ACT, THEN QUESTION. Know your weaknesses and limitations. Work hard to get better and get past them. Don't stagnate by stubborn refusal to get better. The result is much worse than you can imagine and much harder to fix than you'd care to imagine. Fight smaller battles while they're manageable, instead of waiting for a full-blown war with yourself.
Don't get annoyed with people just doing their job. Don't let slide people doing their job poorly. Take them to task.
Don't be embarrassed to ask for your stuff back. Generosity is great, but harmful when abused by others.
Fix problems as they arise. Don't let them fester, big or small. Attack them relentlessly and with gusto.
Do no harm. Do no evil. Make your own slogan and stick with it. Leave no person worse off for having met you or dealt with you. Very hard maxim to follow, but critically important to live up to.
If you undertake to help someone, do it with a whole heart. Don't whine, don't bitch, don't moan. Finish what you promised to do. Be grateful for the opportunity to help someone meaningfully. You will not always be reciprocated directly, and don't expect it. You will be paid back in kind from another source, often a surprising one, often much later.
***FINISH WHAT YOU START*** This is by far one of the hardest resolutions to keep, but the one that separates good intentions (paving the way to hell, some say) from facts on the ground and accomplishments. Most people start far more things than they actually finish. Be the exception that proves the rule.
***Be reliable, ALWAYS. A reliable friend, son/daughter, brother/sister, BF/GF, colleague/teammate, the stranger who helps. Be reliable even if it hurts, if you're not in the mood, if it's inconvenient. It pays off handsomely in gratitude, respect, reciprocation, reputation. Plus, just makes you feel good about yourself. Why not?
***BE THE SAME PERSON ON THE INSIDE AS OUT. Not the same shmuck, But the same decent, good human being - a mensch. Do no harm, do no evil; do lots of good and do well. The whole world will be fighting to make you something or someone else than you are, but your must - and will - persevere.
Develop your systemic thinking regularly and profoundly. Systems of greatest relevance are your mind, your body, your family, your employer, your spiritual universe, the structure and hierarchies and spheres of knowledge and learning, the gaps in your knowledge (including known knowns and known unknowns); mathematical, physical, chemical and biological underpinnings of everyday life, foreign road systems, other political systems, philosophies and forms of spirituality, etc.
You will deal with untold numbers of idiots and incompetent service people until you can afford to pay someone else to deal with them. Be unfailingly patient and polite with each one and try most of all to resolve the issue amicably, if in your favor. Rudeness never helps in the end and only raises your own blood pressure. JUST FOCUS ON SOLVING THE PROBLEM. TUNE OUT EVERYTHING ELSE UNTIL YOU DO.
Don't internalize barbs or insults (or even worse, perceived insults). When your ego is stuck on prestige as its goal, you'll be constantly mired in you own nonsense and won't focus on self-improvement. This will set you back precious time you'll never get back.
Wake up early. You'll get that worm or prize (or whatever it is you seek) eventually if you make it a habit to rise from your royal bed and wake up with a purpose. All the most successful people do.
Don't rely on other people, as a rule. Rely on G-d. That, or be disappointed in even the people closest to you, sooner or later.
Your and all human behavior follows patterns, many of which are well researched by psychology and social science. The better you know yourself, in as great a detail as possible, the easier you will be able to understand other people, what drives then and how to influence them. This goes in the other direction, as well. Learn from the behavior of others to understand yourself better and to adjust your behavior per your own ethical, moral, philosophical and spiritual frameworks.
The better versed you are in human behavior, the better you can tailor yours in your personal life, your job or business. Emotional intelligence is a highly prized and often lucrative "soft skill" that will always be helpful in all your endeavors.
KNOW AND LEARN AND INTEGRATE AN IMPROVE, SO YOU CAN ANTICIPATE AND SEIZE OPPORTUNITIES. Without hard work and a strong will plus measured optimism, even a barrage of opportunities would be in vain.
Always remember: nobody owes you a damn thing in life. You owe your success and upbringing to a group of people you could never repay.
Be grateful for EVERY.THING you have, 3 times daily, at least.
***READ AND WATCH AND INTERNALIZE: Steve Jobs' Commencement Address at Stanford (2005): http://www.npr.org/blogs/thetwo-way/2011/10/06/141120359/read-and-watch-steve-jobs-stanford-commencement-address
***On regrets at the end of life: http://www.collective-evolution.com/2013/04/27/the-top-5-regrets-of-the-dying/
AND, PERHAPS MOST IMPORTANTLY... Don't look so f-ing serious all the time! Young man - smile! Laugh! Smile! Shout! Say inappropriate things out loud! Run for the hell of it! Jump on a bike and speed the hell away, just because! Let your hair grow wild! (that won't last long ;) Grow a beard! Wear funky colors out! Trespass! Rebel in a clever way! Don't be a an old man and spoilsport! Fuck the rules and get away with it! Screw all that advice! ;)