I have learned a lot within my 20 years of life. But in my opinion, these are the most important lessons of all.
1) Always Remember Your Friends Birthdays
People want to feel special on their birthdays. Never forget to say Happy Birthday to those important to you. They will notice your birthday wishes every year and appreciate you as their friend.
2) Call Your Parents / Guardians
No matter what kind of relationship you have with your parents or guardians, make sure to give them a call once in a while. It's hard to remember to when you are so focused on your life and becoming an adult. But, they helped you become the person you are today. Show your appreciation for them.
3) Respect Other People
Regardless of someone's race, class, sexuality, gender, etc... you should always respect them. There is no reason to dislike someone because they look different or have different beliefs than you do.
4) Don't Try To Change Another Person
Accept people for who they are. If not, walk away. You can't change someone to be a perfect fit for you. If you don't like the way they treat you in public or if they tend to tell white lies and it's affecting your relationship you have two options: leave them, or accept them.
5) Never Settle
On the other hand, if someone wants to change you, leave them. If someone tries to control you, leave them. There are too many people settling with someone who isn't good enough for them. You can always do better.
6) It's Okay To Cut Off The Negative People In Your Life
No matter how long you have known someone, whether it was your childhood best friend or your brother, sometimes you have to cut their negativity out of your life. People come into our lives to teach us things about ourselves and they are not always meant to stay. That's okay. Learn to forgive them or yourself for changing and simply let the relationship go.
7) Spend More Time Noticing Your Environments
Take a close look around you. Appreciate the trees blowing in the wind, the sun on your face and the people going about their days in front of you. Be mindful as often as you can.
8) Live In The Moment
Don't worry about things you can't control. Don't think too much about the future. Don't fret about things that have already happened. The past has already happened; let it go.
9) Honor Your Pain
Feel the existence of your sadness and anger. Try to figure out why you feel the way you do. Then, do better next time.
10) Be More Independent
Take classes alone, go hiking alone, go to concerts alone. Enjoy time spent with yourself.
11) If You Don't Feel Mentally Well, Take A Day For Yourself.
It's okay to drop your responsibilities once in a while and take some time for yourself. Spend a day being lazy, catching up on your hobbies and simply recovering your mental state.
12) Take Up A Hobby
Find something that helps you de-stress. Find something that distracts you from your adult responsibilities. Take a breather doing something you enjoy.
13) Never Compare Yourself To Others
You may catch yourself looking at the beautiful blonde girl in front of you and wishing you looked like her. Ignore these thoughts. Take a good look at yourself in the mirror and appreciate all the unique, beautiful qualities that YOU have.
14) Family > Everything
Family members are the only people who will always be there for you. They have been there since the beginning and will be there until the end. You can always count on them to support and love you when you need it the most.
15) When You Find Your Passion: Never Let It Go
Don't ever give up on your passion. Work hard every day to achieve your goals. The things you are passionate about are not random, they are calling you for a reason.
16) Tell Your Friends You Love Them Randomly
Bring your friend a coffee just because. Do the dishes for your roommate when she is stressed out. Call your old best friend and ask about how they've been. Express your love to your friends because you never know what can happen.
17) Random Acts of Kindness Are One Of The Greatest Goods You Can Do For The World
Volunteer often. Pay for the person's coffee behind you. Buy a homeless person a sandwich. Then, don't tell anyone about it. Keep the reward to yourself.
18) Be Positive, Always
The world is beautiful. If you can't see it, keep looking for the things that make you happy.
19) Learn To Love Yourself
One of the most important lessons everyone should learn is to love themselves. You can't love another human being until you take a good, long look at yourself and choose to love everything. Accept your flaws and choose to look at the good qualities you have.
20) Bettering Yourself As An Individual Is The Only Way To Change The World
The world can be a messed up place. Society can be a messed up system. If you want to change the world, the best way to start is fixing yourself as an individual. Be positive and release all the positivity into the universe. The individual self is just as important as the whole. 
Whether it's learning to love yourself or being respectful to others, if you try to be the best you can be life will reward you. The older we get, the more lessons we learn and sometimes they can be hard. Just remember, it's all happening for a reason.
