[Lemmo]

If you are going for definition rather than bulk, you shouldn't need access to a a wide selection of weights. At your age, if you are doing a lot of cardio, you should be fine doing push-ups (focusing on form) and pull-ups, you should be fine. If you invest in a couple of lightweight dumbbells, this will expand your options (e.g., hold them while doing lunges to work the lower body, lift one when at the top of the push up for your back).