What weights should a thirteen year old going 14 lift?

Well i do not wish to hurt hurt my height potential. So what is the maximum weight i can do, and what exercises should i do. I am trying to tone my body, have decent biceps (better than most 13 year olds) and work on my chest a bit. This may sound wimpy, but I only lift up to five pounds since thats what the doc said, yet i have some pretty good results. HE told me to focus on reps, not weights. Any advice?