Summarize:

Nothing communicates maturity like confidence. Even if your physical appearance, unique personality, or social skills aren't what you wish they are, that doesn't have to stop you from building self-confidence. There's a fine line between being confident and being arrogant or condescending. Confidence is feeling good about yourself, not better than those around you. Don't excessively brag about your accomplishments or speak in a way that puts you above others. This is classic immature behavior. Slouching is another classic behavior associated with teens. Hold your head up high and straighten your back. Learn how to walk confidently and improve your posture. Start by standing up straight wherever you are, whether it is walking down the street, sitting at your desk, using a computer, or waiting in line. Correct yourself whenever you notice your posture faltering. Soon, it will become second nature. Changing the way you carry yourself goes hand-in-hand with confidence. Keep your head up high; don't stare at the ground. Look people in the eye when you talk. Speak slowly and confidently instead of loudly. Remember your manners by using “please” and “thank you.” Listen closely to what others say; few things demonstrate maturity as much as a good listener.  When people finish stories about themselves, don't immediately launch into a story about yourself. This makes you look self-centered and uninterested. Instead, react to their story first, then use your own story as a way of relating to the other speaker. Learn how to use small talk. Ask how people are doing. Talk about the weather. Ask about their families. Be polite and carry on light conversation with people. People who are always complaining come off as immature and self-centered. As you mature, you accept that things come and go, and that complaining is unlikely to assist you. Brooding about how awful your life is won't help you actually improve it. Venting to your friends can be beneficial for catharsis or to solicit advice, but constantly complaining about inconsequential occurrences is very juvenile. This doesn’t mean dropping unnecessarily large words during everyday conversation, which will make you look desperate for attention. Focus on cutting out childish words that will clearly identify you as a teen. Speak slowly and deliberately. Make your words count by thinking about word choice.  Learn some sophisticated vocabulary. A good place to start is with SAT vocabulary. For example, say, "That's a novel idea!" instead of "That's a new idea." Tell someone to be "candid" with you instead of honest. A more sophisticated vocabulary makes you look mature and intelligent. Try not to overuse slang. Avoid "like," screeching "Oh My God!," and punctuating sentences with "totally," "you know," and other meaningless fillers. Try not to use "awesome" and "dude." When texting, avoid typing in all capital letters, because it gives the receiver the impression that you are screaming, which is not something that is acceptable among adults in most situations. In addition to that, it is generally a good idea to use profanity in moderation, and avoid using it when it does not provide the receiver a further understanding of what you are saying. If someone disrespects you, feel free to tell them to stop. After all, part of maturity is commanding respect. Be straightforward and communicate what you want. Avoid using excessive sarcasm or acting extremely offended. Even if you're joking with these snarky comments, they make you look bad and won’t achieve the results you’re looking for.  For example, if someone interrupts you, say, "Hold on, please don't interrupt me." Don't say things like, "Wow, I can't believe you just interrupted me. Someone doesn’t know when to shut up!" Learn when to let things go. Everyone has negative encounters with others, but it takes wisdom to learn how to pick your battles. Being spiteful, resentful, or malicious will make you look immature. Learn the difference between venting to a close friend and putting someone down behind their back. Sometimes when people rub you the wrong way, they often don't realize it. Be respectful of their feelings by either ignoring the mistake or, if necessary, clearly communicating to them that they have caused offense.
Be confident. Change the way you physically carry yourself. Develop social graces. Complain less. Improve your vocabulary. Stand up for yourself politely but firmly.