Hey i got a work out question please answer?

well im 13 and i really want a six pack thats noticeable even through my shirt so i started going to the gym i dont lift weights cause i know that could effect my height so instead i go on the tread mill for like 3 to 5 miles and then on that bike machine for like 1-2 miles what is that doing for me and how do i get a six pack and is it not good to do any weights at all?