What are good abdominal work outs for a 13 year old to get a six pack?

i do alot of cartio at the gym cause i live in a pent house and theres a gym just 15 floors down so i run and walk on the tread mill alot! like 3 to 5 miles a day to lose all my fat but i know i need to do abdominal work outs to actually build a six pack so what should i do and remember im 13 and do i really need to watch my diet alot??