I am 15 years old and I want to know what styles would help me look my age, most of my clothes make me look 10?

Im 15 years old, when i see something i like ill buy it, but i never really know what it can go well with, i want to look my age but i dont want to really 'fit in' I like clothes that feel comfortable and are easy to put together e.g. jeans and a t-shirt. however i find it really difficult to find jeans because i am so short and skinny :( im about 5 foot, and very skinny so when i try jeans they never fit in the right places - they would fit my legs fine but not my bum area. etc... I'm into fashion so i want clothes that are in with the latest trends, i also like vintage clothing:) I would say that my style inspiration is somewhere between nicole richie/selena gomez/emma roberts.. i have dark brown hair and my skin is an olive colour. I find it really difficult to find clothes that fit well and go together, im quite casual. I dont want clothes that are really 'girly' and 'preppy' because im not like that.. thank you if you can help! :))