What is the most common problem teens face?

i am writing a book about a teen girl and what she sees throughout her first year in high school. im still in my first year and what i have seen, heard, and done myself shocks me. we all had that period in our life when innocence and curiousity was all we knew and then we enter this completely different relm, thats what im trying to explain so just give me some examples of what you went through. thanks a million