This won't be popular, but maybe you'll read it. It'll be down voted below your threshhold I'm sure.
Unless you desperately want children, like a deep desperate need, don't have them. Resist the urge to get into long term relationships. Resist the urge to move in with someone or have them move in with you as long as you can manage. Unless you, again, desperately feel you need to be in a relationship, avoid it. Stick to having fun. Travel the world. Live cheap as you can and spend time having experiences.
If you're fat, lose weight now as every year will be twice as hard. Never let yourself get fat, your life will be horrible compared to being thin, and you'll lose out on large aspects of life and experience if you fatten up. Exercise every day if all you do is walk. You'll learn and experience more things walking than any mode of transportation.
Do not date or have partners with health problems, physical or mental. They will suck your energy dry. It sounds like a dick move, but you'll end up being their caregiver. Avoid them. Focus on your health and your life.
If you decide to do drugs or drink, makes sure it's fun and social. Don't become someone that drinks alone or uses weed as a daily stress relief. Make it an occasional thing, and it's best if you never do it. Your health will be much better and all that time drinking and doing drugs will be barely remembered and have cost you money and time you'll never get back.
Move frequently. Don't get stuck in one city or even one neighborhood. Between your twenties and forties you change your brain and thoughts on things substantially. Everytime you move you can restart with a new perspective and people around you. You'll accelerate your learning and break bad habits with the past. You will realize what's important in your life and start throwing away crap. Don't let crap build up in your life. Moving allows you to shed it off and keep your priorities straight.
Your social circle will be the most important thing in your life. Your social circle will be a source of jobs, help, resources, friendship, support and so many things. Be careful who you add into it, and be willing to rid yourself of toxic people. Don't think twice about removing friends, family members, anyone who brings you down and makes you feel bad about yourself or your life. Your social circle should be a source of happiness, fun, support, and people you value. Remove those that are burdens. Don't burden them either. Be polite, kind, helpful, and loving, and accept it in return.
Don't feel obligated to anyone. If you owe someone pay it. Favors for favors, friendship for friendship. Value those you let in close. People earn trust, they should never be granted it automatically.
Learn a discipline. Music, painting, sailing, skydiving, fishing, baking, something other than the Internet and Xbox. Where you should exercise your body to keep in shape you should exercise your mind and skill set with disciplines. Most people pick up a hobby and lose it over the years. Practice and become skilled at one or more things. Play the oboe, become a saucier, ice carving, do something that you practice and become better at and challenges you. You never know when a skill helps, and you'll meet different people and expand your knowledge.
if you decide that you aren't the type to travel the world and experience all you can and absolutely love the career route and stability, pay close attention to your appearance. The better you look the further you go. Hygeine, vocabulary, style, clothing, humor, everything will be subconsciously and consciously judged. The right look gets you promoted, while looking slovenly will make you more desirable to get rid of. Perception will be very important. If you go the career route you are more shackled to convention and societal expectations than other options. Learn to navigate it and not be frustrated by it.
Save as much of your income as you can, and invest it conservatively. Avoid thoughts of getting rich quick in the market, it almost never works out, and the real winners are the ones in control of it. The best investment is usually yourself, your health, and your happiness, but save money for the future. Spouses, children, houses, etc. are expensive but manageable. The biggest screw up you'll make is marrying someone that makes a good boyfriend/girlfriend, rather than a good spouse. Know the difference between the two. If you decide to get married and have kids, fundamentally change your life for that. Don't think you can go party all night and occasionally go home at 2am. It won't work out. Put yourself in the mind and actions of a married parent.
Good luck. You can always just make it up as you go. That's what most people do, just jump into adulthood with no planning whatsoever.