Okay, as far as the acne, I see you're getting a lot of suggestions on how to handle that, and it will eventually start to go away. By way of hairstyles, a short haircut is fine, as long as it doesn't look too young. Basically, it will be the type of hair you see older people have, that teenagers are embarrassed to have. For clothes, try and ditch any loose pants (pants on the ground kind) and baggy shirts. Get some nice t-shirts that actually fit you, (nothing too immature on it) and some jeans/khaki's/etc. If you want to change it up some, get some nice polos for a more professional look.
Maybe you can try some hollistic treatments for your acne if traditional methods have failed, that will help a lot. Maybe you can work out and gain a little muscle, that tends to add some age. I think if you keep well groomed and wear just a normal tee-shirt and jeans you should be OK. Remember, stand up straight, carry yourself well, and it will also make you look more like an adult. Good luck.
Put lemon juice on affcted acne areas.continualy till it gets alright.and yes older people talk like elders walk like elders ...and they look like elders.grow mustach ,make some muscles and have a responsiblity taking personality.you will look and feel older.
Trust me pal in the future you will be one very happy person if you look young for your age! I'm 35, but I get told by many that I look in my twenties. When I was your age I couldn't even get into pubs that easily, I was always asked for i.d. I hated it then but now I'm just happy I look young for my age. My point in all this is, don't go over the top to change. Yeah expirment with hairstyles for your age group, grow a goatee (I did this and it wasn't the best but I loved it lol), just act your age and that's the most important thing.