Talk about how choices become habits. Habits can become addictions. Describe how pathways are literally dug into the brain and it's very hard to change them once those paths are made.
If you see a kid sitting alone, he or she might be your next best friend. Go say hi! Ask him/her to sit next to you / play on your team, etc. No need to worry about whether that person is “too cool” or “not cool enough,” they’re just a kid. Like you.
Read BIOGRAPHIES. So many interesting stories and valuable lessons. Historical figures, those close to them (those are often the real treats!)
Also people from all walks of life, different industries. Artists, entrepreneurs, athletes, statesmen, etc. Bonus points for listening to the audiobook if subject narrates it.
Major life events such as the birth of a baby, graduations, new job, (lost job) are a big deal. Make a fuss. Take an interest in loved ones’ experiences. Acknowledge their feelings. Express a sincere interest in others (young and old) will profoundly affect your relationships.
Kindness is key. Practice being kind even when you don't feel like it. Let's all encourage each other to be kind always. (Or learn how to take a walk or go to our room when we just can't be kind in that moment.)
If you feel like you can’t get your act together, that you’re less mature than some others your age, I have the recipe for being a grown up: Read, travel, help those in need.
READ everything you can get your hands on: books, audio books, newspapers, etc. Travel to places that are different than your experience. Keep a lookout for people (or animals, or groups or causes, etc) who are struggling. HELP those in need.