Adults, looking back, what advice can you give to an 18 year old that you wish someone had told you?

Don't believe in what anyone else tells you, unless you believe in it yourself. You are yourself, you can always take advice, but you are the individual who will take life by its reigns. When others mock you for what you believe in, laugh with them. Understand that every single person has a completely dif set of likes and dislikes, and tolerance is the first step towards understanding. Mocking someone's taste is no different then to tell them they're stupid. Understand that almost nothing is certain. And nothing will ever go according to plan, so always be ready. Find life's meaning for yourself, everyone sees the world through different eyes. There is always black and white, but the shade of grey we see the world in, is what makes us us.

Hope any of this is meaningful in the least

Some advice that I was given but didn't really listen to was: If a guy you're interested in dating is over the age of 30 and still lives at home with his mother, run the other way! Another piece of advice that's useful is respect yourself enough to wait. Even if you've already made the mistake of doing it before that doesn't mean you have to continue making the same mistakes over and over. Reclaim yourself and really make the next time you decide to do it something worth waiting for. Any guy who gets mad about not getting it, it's worth giving it to anyways. The "one" will respect you enough to wait. Also, I would like to throw in if you are not going to wait, be mature enough to take safety into your own hands. Don't assume it's just his responsibility. You don't expect him to buckle your seat belt for you every time you get in a car then don't expect him to do that by himself either. The future is at your feet and is yours for the taking. Sometimes one seemingly small decision can alter the course of the rest of your life, so choose wisely. : )

failure is learning. when some one tells you No, be thankful and move ahead. if friends don't have your schedule don't change you own to meet their expectations. money is everything, careful it can be misleading.