Tshirt ideas!?!?! help?

okay, so i have a tshirt business online, and i have sold a few shirts, but i want them to be very popular and make lots of money (: i either want a new macbook pro or like a new jet ski ( since my parents wont buy me a new one lol:( or like a range rover ha .... so i want them to be VERY SUCCESFULL anyway um got any ideas, you know what do teenagers want to have on there shirts?