I had the good fortune to be mentored by an older DM who was willing, during the summer, to be a DM for us munchkins. Seeing how older players play is one of the best things for a younger player. See whether an older player - someone's cousin or older sibling, a parent with vast experience - is willing to run AP, a short campaign, or a one off for you or you and a small group of peers.
If a group of older players is willing to take you in, then don't be afraid to contribute. The most important step a novice RPer can take is overcoming their nerves and their feelings of embarrassment and speaking in character. However, I would advise you as the munchkin in a group to approach play humbly and soberly, and to try to adopt the goals and style of the group as your own. Try not to live up to any negative stereotypes and avoid in your excitement or pleasure of the game losing control of your emotions or reason. Likewise, avoid the negative stereotype of being always creating mischief for the group. On the other hand, don't let them boss you around. Be your own character and take as wise of actions as you can. Indeed, I would advise creating an initial character that is inspired by characters of fiction that are mature and suave - Spock, Obi Wan Kenobi, James Bond, Aragorn, etc. - and try to live up to that. Don't make a joke or jokey character (unless everyone in the group does, and that's the groups play style). Don't make a character that is a pure pastiche, but talk with the DM about making a character that suits his world. That way, you'll have no excuse for indulging bad habits. Be a reliable partner in play, and they'll soon forget your age and you'll find yourself with a niche in the party.
Honestly, one of the better starting characters for a young player is a cleric. Not many players want to be the cleric, but everyone needs the cleric. Playing a cleric with a philosophy that the group appreciates and who though serious isn't a pushing stereotypical moralist is a good way to win some respect. If I knew your game's setting, I could make suggestions. Or, if you already have a character, make sure you play the character maturely.
In practice though, you are probably going to enjoy the game best with your peers. Be looking around for fellow nerds that want to play, and who might find the ET or Stranger Things experience cool. In many ways, the gaming you do before you leave High School is - though it won't be the most mature or sophisticated you'll do - will be some of the most fun and memorable.
PS: I've been running games since before you were born. Some of the most mature, intelligent play I've seen was done by 13 and 15 year olds taking the game and its conceits seriously. Whereas, on the other hand, the group of middle aged players I currently run frequently acts like they are 13. Don't let anyone tell you a young player can't play the game and play it well. You have a great advantage that they don't have - you quite recently have experience playing make believe and otherwise engaging in play. You probably take your play more seriously than they do, as the young understand what adults have forgotten, that all play is practice for living.