One of the most frequently asked questions that I've heard in my years of training! Don't be afraid to start and just work on your form for weight training but also don't forget that push-ups and body-weighted dips on the edge of your couch will not hurt. Herschel Walker used to do 5,000 push-ups and 5,000 sit-ups every day and look where he ended up... Arnold started lifting at a young age as well and he towered over most of his bodybuilding competition and the normal population! But, do not* load up the weight early on and try to set Olympic records! Enjoy the process if you're young and set up a nice base for yourself in the future; trust me, you'll thank yourself that you did!